[TwigBundle] show correct fallback exception template in debug mode
bug #12196 [TwigBundle] show correct fallback exception template in debug mode (Tobion)

This PR was merged into the 2.3 branch.

Discussion
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[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.

Commits
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b97acd9 [TwigBundle] show correct fallback exception template in debug mode