{Pairing, Authentication}Status.__getattr__ raises KeyError instead of AttributeError

We use `__getattr__` to make accessing the response data nicer, so `__getattr__(self, name)` returns `self._raw_data[name]`. However, if `name` isn’t in `_raw_data`, a KeyError will be raised. I think most Python programmers would expect an AttributeError. We should probably also support nested attributes, because `auth_request.requester.id` is way less pretty than `auth_request.requester['id']`.
