Making default unicode type work with required arguments for "json" location.

Argument `convert` method was raising an error when trying to convert a required argument to the default `unicode` type. This at least was happening for `json` location. 

I added `LookupError` to the list of handled exceptions, however it might be not the best way. `Argument` documentation mentions `ValidationError` that should be raised by the type converter but this class does not exist in the repo: https://github.com/twilio/flask-restful/blob/master/flask_restful/reqparse.py#L34