connection: refetch token if invalid.

https://developers.google.com/drive/web/handle-errors#401_invalid_credentials

If the API returns a 401, this will nullify the token we have and fetch a new one before trying the request again.
Kind of just the convention. If we had multiple error handlers for success and error, I would use `onAuthorizedSuccess` & `onAuthorizedError`, but instead, I'm removing the specificity by saying `onAuthorized[Event]`. We use it below with `onConnected` as well, even though the connection can return an error.

All that being said, I'm always happy to change names if it makes it more understandable. Any ideas?