[TwigBundle] Fix error page preview for custom twig.exception_controller

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12147
| License       | MIT
| Doc PR        | n/a

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.

(The other approach would have been to fiddle around with `ExceptionController`'s `debug` flag through a setter when going through the preview action, but that would have been even more messy.)
should be ``$request->attributes->get('showException'`` actually