Make error message in controller translatable.

At the moment the error message that is placed in the controller seems not to be translated. 

The message is copied from the flash messenger into to the form messages. However the FormError renderer that is used by default does not translate messages. Hence no translation.

This PR fixes this by adding the `Translator` as an optional dependency of the `UserController`. If present it is used to translate.
@roelvanduijnhoven Suppose, the site I am developing is not multi-language based. In that case, if this property is kept, we can easily override the message by extending the controller(ofcourse). I don't need to worry about translation in that case.