[FrameworkBundle][Translator] Validate locales.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
I'm +1 for adding @throws in [FrameworkBundle Translator](https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Translation/Translator.php) class but not in the interface.