Symfony - No Extension Is Able To Load The Configuration (Subscriber/Event Listening) (REST API Exception Handling with JSON output)

Question

Purpose

Hello, I am fairly new to Symfony and am trying to create an exception/error handling functionality for our Web API.

Requirements

When a user makes an invalid request to our API, we want to return a simple JSON object that looks something like the following:

{"errorCode": 1234567, "errorMessage": "Parameter 1 is invalid. String expected"}

In addition to API-specific errors, we also want to hook into the built-in exception-handling in Symfony and re-use that, but instead of returning an HTML error page (which is what happens by default), we want to return a JSON object like:

{"errorCode": 404, "errorMessage": "Not found"}

Current Approach

Obviously, we want this to be implemented in the most efficient way, so after doing some research, I found what I think is a great approach which I can then adapt to our specific needs. Here is the tutorial I followed:

https://knpuniversity.com/screencast/symfony-rest2

I ended up buying this course in order to get access to the full code. The issue is that it was written for Symfony 2, but we are running Symfony 3.3, so some things are outdated and I have not been able to get the example running yet.

Code

So, I have posted some of the relevant code below. (I obviously cannot post the whole code, but have posted what are hopefully the relevant portions of the publicly-available code).

services.yml (Their version, Symfony 2 Style)

api_exception_subscriber:
        class: AppBundle\EventListener\ApiExceptionSubscriber
        arguments: []
        tags:
            - { name: kernel.event_subscriber }

services.yml (my full file with all comments removed) This includes a modified version of what they have above (hopefully this is the correct Symfony 3.3 way of doing things). NOTE: Anything that was part of the private code which I cannot show, I have replaced with the word "something".

parameters:

services:
    _defaults:
        autowire: true
        autoconfigure: true
        public: false

    AppBundle\:
        resource: '../../src/AppBundle/*'
        exclude: '../../src/AppBundle/{Entity,Repository,Tests}'

    AppBundle\Controller\:
        resource: '../../src/AppBundle/Controller'
        public: true
        tags: ['controller.service_arguments']

    AppBundle\EventListener\ApiExceptionSubscriber:
        arguments:
        - ['%something.something%']
        tags:
        - {name: 'kernel.event_subscriber'}

routing.yml (relevant section of their file)

app_api:
    resource: "@AppBundle/Controller/Api"
    type:     annotation
    defaults:
        _format: json

routing.yml (my full file) - NOTE: I had to add the "_format: json" directly to the "app" section here rather than "app_api" because in our REST API, all of our URLs are at the root level, and do NOT have to be prefixed with "api" like http://localhost/api/someMethod/someMethodParameter as in the tutorial's code

app:
    resource: '@AppBundle/Controller/'
    type: annotation
    defaults: {_format:json}
yml_route:
  path: /yml-route
  defaults:
    _controller: AppBundle:Default:yml
awesome_route:
  path: /awesome-route/{ic}
  defaults:
    _controller: AppBundle:Rest:awesomeRoute

src/AppBundle/RestController.php (edited to show just the basic structure)

    <?php 
    namespace AppBundle\Controller;
    use FOS\RestBundle\Controller\Annotations as Rest;
    use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
    use Symfony\Bundle\FrameworkBundle\Controller\Controller;
    use Symfony\Component\HttpFoundation\JsonResponse;
    use Symfony\Component\HttpFoundation\Response;
    use Symfony\Component\Serializer\Serializer;
    use Symfony\Component\Serializer\Encoder\XmlEncoder;
    use Symfony\Component\Serializer\Encoder\JsonEncoder;
    use Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer;
    use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;
    use Symfony\Component\HttpFoundation\Request;

    class RestController extends Controller {

       public function doSomething($id)
       {
          // Code to populate $result with data from database is here
          return new JsonResponse($result);
       }

    // This file contains many different functions similar to doSomething() which retrieve the request for the API caller.
   // For example, doSomething() can be accessed via http://localhost/app_dev.php/doSomething/123
    }

src/AppBundle/Api/ApiProblem.php (edited to show just the basic structure)

namespace AppBundle\Api;
use Symfony\Component\HttpFoundation\Response;

class ApiProblem
{
   // Code relevant to this class is in here
}

src/AppBundle/Api/ApiProblemException.php (edited to show just the basic structure)

    <?php

    namespace AppBundle\Api;
    use Symfony\Component\HttpKernel\Exception\HttpException;

    class ApiProblemException extends HttpException
    {
       private $apiProblem;

       public function __construct($severalParametersWhichICannotListHereBecauseCodeIsPrivate) {
       // Does some stuff and then calls the parent:__construct
       // Includes a getter for $apiProblem
    }
}

src/AppBundle/EventListener/ApiExceptionSubscriber.php (edited to show just the basic structure)

<?php 
namespace AppBundle\EventListener;
use AppBundle\Api\ApiProblem;
use AppBundle\Api\ApiProblemException;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpKernel\KernelEvents;

class ApiExceptionSubscriber implements EventSubscriberInterface
{
   // Code relevant to this subscriber is here
}

The Issue

I'm getting the following error when I try to run any type of command from the command-line bin/console:

    C:\htdocs\projects\myproject>php bin/console debug:container
    PHP Fatal error:  Uncaught Symfony\Component\DependencyInjection\Exception\InvalidArgumentException: T
    here is no extension able to load the configuration for "AppBundle\EventListener\ApiExceptionSubscribe
    r" (in C:\htdocs\projects\myproject\app/config\services.yml). Looked for namespace "AppBundle\EventListe
    ner\ApiExceptionSubscriber", found "framework", "security", "twig", "monolog", "swiftmailer", "doctrin
    e", "sensio_framework_extra", "demontpx_parsedown", "doctrine_cache", "doctrine_migrations", "debug",
    "web_profiler", "sensio_distribution", "web_server" in C:\htdocs\projects\myproject\vendor\symfony\symfo
    ny\src\Symfony\Component\DependencyInjection\Loader\YamlFileLoader.php:644
    Stack trace:
    #0 C:\htdocs\projects\myproject\vendor\symfony\symfony\src\Symfony\Component\DependencyInjection\Loader\
    YamlFileLoader.php(614): Symfony\Component\DependencyInjection\Loader\YamlFileLoader->validate(Array,
    'C:\\htdocs\\proje...')
    #1 C:\htdocs\projects\myproject\vendor\symfony\symfony\src\Symfony\Component\DependencyInjection\Loader\
    YamlFileLoad in C:\htdocs\projects\myproject\vendor\symfony\symfony\src\Symfony\Component\Config\Loader\
    FileLoader.php on line 179

    Fatal error: Uncaught Symfony\Component\DependencyInjection\Exception\InvalidArgumentException: There
    is no extension able to load the configuration for "AppBundle\EventListener\ApiExceptionSubscriber" (i
    n C:\htdocs\projects\myproject\app/config\services.yml). Looked for namespace "AppBundle\EventListener\A
    piExceptionSubscriber", found "framework", "security", "twig", "monolog", "swiftmailer", "doctrine", "
    sensio_framework_extra", "demontpx_parsedown", "doctrine_cache", "doctrine_migrations", "debug", "web_
    profiler", "sensio_distribution", "web_server" in C:\htdocs\projects\myproject\vendor\symfony\symfony\sr
    c\Symfony\Component\Config\Loader\FileLoader.php on line 179

    Symfony\Component\Config\Exception\FileLoaderLoadException: There is no extension able to load the con
    figuration for "AppBundle\EventListener\ApiExceptionSubscriber" (in C:\htdoc
    fig\services.yml). Looked for namespace "AppBundle\EventListener\ApiExceptio
    work", "security", "twig", "monolog", "swiftmailer", "doctrine", "sensio_fra
    arsedown", "doctrine_cache", "doctrine_migrations", "debug", "web_profiler",
    eb_server" in C:\htdocs\projects\myproject\app/config\services.yml (which is b
    ocs\projects\myproject\app/config\config.yml"). in C:\htdocs\projects\myproject\
    \Symfony\Component\Config\Loader\FileLoader.php on line 179

    Call Stack:
        1.4353    6149416   1. Symfony\Component\Debug\ErrorHandler->handleExcep
    myproject\vendor\symfony\symfony\src\Symfony\Component\Debug\ErrorHandler.php:

What I've Tried

I've been debugging this for the last several days and have read many related discussions on Stack Overflow. I'm fairly new to services, subscribers, and dependency injection and I've tried editing both the routes.yml and services.yml numerous times. I was also getting some circular reference exceptions before but think I've fixed those now. I was hoping someone could please provide me with some direction and hopefully help me turn this into a working example on Symfony 3.3 that I can learn from. If you need any additional detail, please let me know.

From what I've learned, autowiring/autoconfiguring of services in Symfony 3.3 seems to be new and I think that may be impacting things but I'm not sure. I did try turning both of those settings off in services.yml though but with no luck.