deposit.auth: Fix authentication failure corner case
The error raised from keycloak can be less detailed than anticipated. This makes the deposit server fails. So this commit fixes it by detailing only if it can. Delegating such details to the newly craft swh.auth.keycloak.keycloak_error_message utility.
Related to swh/infra/sysadm-environment#3166 (closed)
Test Plan
tox
Migrated from D5311 (view on Phabricator)