Add token not loaded callback

Add new `tokenNotLoaded` callback. Useful for knowing when an `autoLogin` or` autoLoadStoredToken` request was not successful. My use case is I want to show a loading animation until this library figures out if an automatic login was sucessful or not and then either show a login button (`tokenNotLoaded` callback) or communicate with the server and then show an account menu (`tokenLoaded` callback).
_tokenLoaded -> _tokenNotLoaded?