Flesh out authentication examples to handle failure cases

[ndarilek]

Thanks for this library! Trying to use it for the first time, and I'm having trouble figuring out how to detect and respond to authentication/registration failures. Wondering if the examples might be filled out to show this?

I'm trying to check for errors, but none seem to be generated if I enter a non-existent email address/password. Obviously I can check authenticated, but I'm still uncertain how to distinguish between the unauthenticated vs. error case.