Ensure display of Authn errors

In SF 6, the new exception handler does not show the exception message
in the exception handler. Rather, these errors are handled at another
point (authn listeners).

Also see: https://stackoverflow.com/questions/40174266/not-all-exceptions-catched-within-the-error-handler

src/Surfnet/StepupSelfService/SelfServiceBundle/Exception/MissingRequiredAttributeException.php

@@ -20,8 +20,6 @@
 
 namespace Surfnet\StepupSelfService\SelfServiceBundle\Exception;
 
-use Symfony\Component\Security\Core\Exception\AuthenticationException;
-
-final class MissingRequiredAttributeException extends AuthenticationException
+final class MissingRequiredAttributeException extends RuntimeException
 {
 }