properly handle exceptions in translator listener

The `setLocale()` method can throw an `\InvalidArgumentException` when
an invalid locale has been passed. These exceptions must be handled by
the `TranslatorListener` which should then pass the request's default
locale instead.
bug #12181 [HttpKernel] properly handle exceptions in translator listener (xabbuh)

This PR was merged into the 2.6-dev branch.

Discussion
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[HttpKernel] properly handle exceptions in translator listener

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

The `setLocale()` method can throw an `\InvalidArgumentException` when
an invalid locale has been passed. These exceptions must be handled by
the `TranslatorListener` which should then pass the request's default
locale instead.

Commits
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d3a0a55 properly handle exceptions in translator listener