[TwigBundle] Fix that error page preview does not work for custom twig.exception_controller

The testErrorPageAction() always used the showAction() of TwigBundle's ExceptionController.
You can, however, configure an alternate controller by setting twig.exception_controller.

Thus, in order to get a proper preview, we need to forward to this configured controller (which
may be the default one).

This requires us to pass an additional parameter to ExceptionController::showAction to be able to
get the *error* page even if configured otherwise in the constructor.