django/backends: Improve error message when a bearer token expired
This replaces the Keycloak error message by a more comprehensible one for end users.
Unfortunately, there is no way to get the bearer token validity period apart using Keycloak Admin REST API but we clearly do not want to query it in a django auth backend.
Related to swh-web#3121 (closed)
Migrated from D5431 (view on Phabricator)