See discussion at #64

If a null ErrorListener is passed to a Request, Volley just silently doesn't deliver errors. (It previously would crash for a null Listener, which is up to the Request subclass, but that crash is fixed per that pull request). I can't think of a valid scenario where you'd want to pass in a null listener, so we should clean this up everywhere and consider failing more loudly if a null listener or error listener is provided, at least in the classes Volley provides.