gluster: fix forwarding vdsm errors

set 'null' to VdcFault.Error to make the frontend to
show the error returned by vdsm instead of
"Internal Server Error".

Change-Id: I5df008d6626da5ff0de4ad6231ef65f51e08ec84
Bug-Url: https://bugzilla.redhat.com/956159
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>