deposit.auth: Fix authentication failure corner case

The error raised from keycloak can be less detailed than anticipated [1]. This makes the
deposit server fails. So this commit fixes it by detailing only if it can. Delegating
such details to the newly crafted swh.auth.keycloak.keycloak_error_message utility.

[1] https://forge.softwareheritage.org/differential/revision/edit/5311/

Related to T3166