check if I stayed in the same page or not

Question

I use mink to test a website and I want to write a test scenario to check for error scenarios. So I want to see If I insert invalid data a page should return error and I should be in the same page as before. I mean I don't want to go to another page. for that I wrote a test scenario like below:

some steps ....
And Save current page
some other steps....
Then I should stay in the same page

for the two steps I wrote to method like beloww:

/**
 * @Given /^Save current page$/
 */
public function SaveCurrentPage() {
    $this->_last_page = $this->getSession ()->getCurrentUrl ();
}

and

/**
 * @Given /^I should stay in the same page$/
 */
public function StayInSamePage() {
    if ($this->_last_page === null) {
        throw new FailureException ( "current page is not defined" );
        // throw new Exception ( "current page is not defined" );
    }
    if ($this->_last_page !== $this->getSession ()->getCurrentUrl ()) {

        throw new FailureException ( "you should be in " . $this->_last_page . " but you are in  " . $this->getSession ()->getCurrentUrl () );
        // throw new Exception ( "you should be in " . $this->_last_page . " but you are in " . $this->getSession ()->getCurrentUrl () );
    } else {
        return;
    }
}

But I really don't like my solution. Since the line Save current page does'nt relate to the behavior of the application and is only used for mink and I think is not what behat/mink's goal is. Is there any better solution?
Thank you in advance