Processing of server roles should ignore errors.EmptyResult
    
    When non-admin user issues a command that utilizes
    api.Object.config.show_servroles_attributes(), some server roles might
    return errors.EmptyResult, indicating that a role is not visible to this
    identity.
    
    Most of the callers to api.Object.config.show_servroles_attributes() do
    not process errors.EmptyResult so it goes up to an API caller. In case
    of Web UI it breaks retrieval of the initial configuration due to ipa
    config-show failing completely rather than avoiding to show available
    server roles.
    
    Fixes: https://pagure.io/freeipa/issue/7452
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>