backend: unlock images when GetImageInfo fails

In BaseImagesCommand.endSuccessfully() we try to update the image
information contacting VDSM. If VDSM is down or for any other error
we should fail gracefully (logging) and release the locks on the
image.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1018876
Change-Id: I980e60ba5ad4fcc07752d065c3eaca03af025896
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>