shareVolumeRollback() should raise when recovery failed.

Engine does not care whether an exception is thrown during
rollback, as such exception is caught by task.Task._run() when
it is calls the rollback function.
Removed try blocks in order to raise exceptions caused by failed
volume link removal in other than an inexistent link.

Change-Id: I8f01ff2d30ffecc05b9cfd4978f79003a8032270
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/11422
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>