Ignore exceptions from undefined handlers when using the guard tag

[fabpot]

Fixes #4505

[javiereguiluz]

Thanks for merging this and for releasing it as part of Twig 3.18.0.

Sadly, this change didn't fix the issue for me. See the failing tests:

https://github.com/EasyCorp/EasyAdminBundle/actions/runs/12550268707

[xabbuh]

The reason for the failure Javier observes (and also the reason for #4528 (comment)) is that the following subparseIgnoreUnknownTwigCallables() in GuardTokenParser also throws a SyntaxError.

[javiereguiluz]

Maybe we should reopen this issue to not forget about it. Thanks.

[stof]

I found the root cause of the issue.

Twig/src/ExpressionParser.php

Line 799 in 6513bc4

if (!$function = $this->env->getFunction($name)) {

relies on getting a falsy value for non-existent functions before deciding to ignore them or to fail. To fully support the case of undefined handlers throwing a SyntaxError to provide a custom message, we also need to catch exceptions in that case (this was not caught by tests because this PR did not implement a functional test covering that case but only a unit test, which does not catch that it handles only half of the places that need to be handled).
I will try to do a PR with the missing part.

@javiereguiluz we cannot reopen because this is not an issue. It is the merged PR implementing the change.