Ensure user gets notified if session expired

[diocas]

If the auth session Is invalidated (i.e due to expiration of the SSO), we need to ensure that this is detected and reported to the user with a clear warning and by preventing any changes to a file.

To check:

• [ ] is this still the case after moving to oAuth (since we no longer have a decoupled reverse proxy in front and everything is done by Jupyter/Hub)?
• [ ] How does Jupyterlab react to this?