Improve invalid token handling

[coolya]

If the client sends a token that the server does associate with a user it returns a 401 to the client.

The client will present a generic error message that it couldn't create the gist. Ideally we want to client to handle this in a smarter way:

If the client has stored user credentials it is likely that the token it is using got revoked. Most likely be cause the user delete it form their settings. In this case the client should show a notification that prompts the user to start the login workflow from the client to get a valid token.

As a second action the notification should allow the user to retry to upload the gist e.g. when they completed the login workflow.