Return meaningful errors when the user has not backend services tokens

[lorenzo-cavazzi]

Description

It may happen that users have a valid KeyCloak authentication token but no backend tokens.
This happened recently in a couple of cases:

• GitLab was restarted and the tokens were not valid anymore
• A user tried to use the KeyCloak authentication without redirecting the users to /api/login

The Gateway triggers an exception and the APIs invoked respond with a 500 error. Details in Sentry:

• https://sentry.dev.renku.ch/organizations/sentry/issues/183
• https://sentry.dev.renku.ch/organizations/sentry/issues/200

Reproduce

The swagger endpoint simplifies reproducing this.
Log out from any dev deplyoment, go to the swagger page (<URL>/swagger) and authorize with pkce. Then try to invoke any API on the core service.

Other info

Handling that case properly may require changing the UI server as well to add the headers marking the response as "auth-invalid". That will automatically trigger a re-login in the UI client

[rokroskar]

relates to / is a duplicate of #553