Allow to define multiple message keys in ValidationError

Follow up on the pull request [https://github.com/playframework/Play20/pull/1140].

I missed adding multiple keys in the scala object ValidationErrors.

To limit the impact i didn't change the serialization of the JsError, the Json generated contain only the last key.
For reference the plugin https://github.com/julienrf/play-jsmessages support alternative message keys.