The previous example with just_raise_that_exception method
stopped working because MissingTranslation is not an instance of
Exception class any more, but has a +to_exception+ method.
Also the cleaner way is to re-raise only the desired exception, passing
everything else to the default ExceptionHandler.
Additionally this re-raising conflicts with Pluralization backend thus
we have to add a check that certain missing translation keys should
be ignored allowing the backend to fall back to the default
pluralization rule.
