Give alreadyrolled error when rollback will be a null edit

Give alreadyrolled error when rollback will be a null edit

When a rollback will not create a new version, the rollback should not
be success normally. Raise the alreadyrolled error to indicate this
situation.

This can happen, when the same user already rolled back his edit in the
meantime.

Change-Id: I953ae1ca79abb37bc44f3f8244a42a34111e6ab8