[TwigBundle] show correct fallback exception template in debug mode

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

In debug mode, the exception controller did not show the correct template in case the format is unknown.
It falls back to html but the logic for `exception_full` was not applied there. So it was just returning the partial html.