[CBM] Log errors when trying to unlock object storage lock file

When using object storage we protect the archive with a lock to stop multiple instances of cbbackupmgr (including running on different machines) operating on the archive. We then defer unlocking the archive - e.g. in config - but we don't check the error.

This makes sense as there isn't much we can do with this error, other than perhaps retry the unlock. We should log this error however, as it will help explain cases where there is an unexplained lock file hanging around.