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.
[1] https://sentry.softwareheritage.org/share/issue/ae84f6a49ded4adc84c3570020b527ee/
Related to T3166