core: VmReplicateDiskStart - catch exception on revertTask

VmReplicateDiskStartTaskHandler:
catching exception on 'revertTask' in order to ensure disk status
update (unlock the disk) and to prevent further rollback.

Change-Id: I50dfae1e3903e6fabb8e8f17c9251977d7b2c874
Bug-Url: https://bugzilla.redhat.com/970889
Signed-off-by: Daniel Erez <derez@redhat.com>