volume: remove unneeded rebaseVolumeRollback logging

This patch removes some unneeded extra logging, tasks failures are
already logged by the task framework in Task._run().

Change-Id: I475102c989405870c6f9eb2933c433631ce6e8d8
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26900
Reviewed-by: Dan Kenigsberg <danken@redhat.com>