Yeah!! I got it! Automatically!
Here is my solution:
1- Similar as described by @Grokcodile: edit the project config/environments/development.rb file and add the following line of code to the Rails.application.configure block.
BetterErrors::Middleware.allow_ip! ENV['TRUSTED_IP'] if ENV['TRUSTED_IP']
config.web_console.whitelisted_ips = ENV['TRUSTED_IP']
2- At the Cloud9 edit the ~/.bashrc...
vi ~/.bashrc
add the line (enter, alt+a):
export TRUSTED_IP='0.0.0.0/0.0.0.0'
Save it (esc, :wq)
3- run rails s -b $IP -p $PORT
as usual...
4- Enjoy better errors!!
If you also work on this project at a Virtual Machine(vagrant):
1- edit at your VM (vagrant) your ~/.bash_profile (my case) and add:
export TRUSTED_IP=x.x.x.x
export PORT=3000
export IP=0.0.0.0
x.x.x.x must be equal to the REMOTE_ADDR of ENV. (This in not a problem like cloud9 because at my VM the IP doesn't change everytime: 10.0.2.2 always for me).
With this I am now able to use the gem foreman: foreman start
at both places with the Procfile:
web: rails s -b $IP -p $PORT
This works because the global env variables are set on both.
I am just starting to learn RoR too, so, hope this is the right thing to do without bringing more problems in the future.