Add - failWithError to WVPResponse to allow proxying network failures

Using the previously available `respondWithError:text:` method, it's not possible to proxy network failures through to the web view as expected. By adding a method to call the client's `didFailWithError:` method, we can simply passing the connection error right on through, and things behave as expected.

I didn't remove the other method, because that's a backwards incompatible change and figured you'd want to handle that in your own time.