title: How to disable redirection after login_check in Symfony 2
body: <p>I need to disable redirection after login check, because I need to get only that the login was success or not. After submission /login_check url give me the right data, but keep redirecting to /login (on failure).<br/>
  /login is blank after that.<br/>
  I am trying to set up login form using extjs 4 so I need to validate trough an ajax post request.
  login_check should authenticate, create user session and return whether it was success or failure, but no forwarding anywhere.</p>
  
  <p>my login.html.twig looks like:</p>
  
  <pre><code>{% if is_granted("IS_AUTHENTICATED_REMEMBERED") %}
      { success:true }
  {% else %}
      { success: false }
  {% endif %}
  </code></pre>
  
  <p>and in security.yml:</p>
  
  <pre><code>firewalls:
      main:
          form_login:
              provider: fos_userbundle
              failure_path:  null
              failure_forward: false
  </code></pre>