This changes the default behaviour to include `self.client_token`
when using `authenticate`. If `self.client_token` is `None`, a new
token is generated using uuid4 (like the vanilla client does).
- Return value of _make_request() is treated as a requests.Request, when it is in fact a requests.Response.
- Some tests in test_authentication use assertRaises() incorrectly, resulting in testing code that never gets run.
- Other miscellaneous errors exposed by the above changes.
Additionally:
- YggdrasilError instances now have fields with specific error information, and _raise_from_response() populates them. (This will be useful for later changes.)
