Do not crash if group name of group cannot be found
Do not crash if group name of group cannot be found

(cherry picked from commit 01947d84c25a17462305f943594ed1b5d099f4b2)
(cherry picked from commit ed323694d38e6722ab98042158eb98e2640d6e6f)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend