[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.)
Yes, we try to avoid Bc breaks whenever possible. So, the old name should be kept.