More complete AR error handling

[nbrooke]

As mentioned in the comments on #566, we don't handle the ARSessionDelegate failure callback right now (https://developer.apple.com/documentation/arkit/arsessionobserver/2887453-session). The most common error (no camera permissions) is handled by that pull (but could be handled through that), and the configuration invalid error should never happen (because we preflight check the configuration validity and won't even show the button), but there may be some user controllable situations where the other ones happen (do we get a world tracking failure if the room is just to dark to get any fix?), and at the very least we should show a "something unknown went wrong" if we do get one of those.