[FrameworkBundle][Translator] Validate locales.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
@fabpot this means that you every method can throw any exception, indepently what is documented via `@throws`? Then we can remove the throws documentation completely because a developer much catch any exception on any method anyway.