title: Error message on registering registered user in fosuserbundle
body: <p>Is there any way to show a flashbag message into <code>FOSUuserBundle</code>'s <code>register.html.twig</code> once the user has already registered but is trying to register again?</p>
  
  <p>Example:</p>
  
  <blockquote>
    <p>This username/email is already registered, please check your mail or register with another account</p>
  </blockquote>
  
  <p>What code do I need to include in <code>RegistrationController.php</code>?</p>