add overriable method for Callback.onFailure

[lucianobustos]

Some times the json service return erros and/or wrong serialized data. You need to be able to handle that unexpected response in the onFailure CallBack.

api.getProdByName(name, new Callback<Response>() {

        @Override
        public void onFailure(Exception ex) {
            //some nasty failure happened here
        }

        @Override
        public void onResponse(Response<Response> response) {

        }
    });