start a scout if remote leadership fails

when handling a leadership change, it is possible that sirius can fail
to resolve a remote reference to a new leader. This has been observed
during rolling restarts and on commodity hardware. If this happens and a
scout is not started, the node's leader will never resolve (ie remain
unknown) and will be unable to participate properly in its cluster.
It's not that we necessarily need an Unknown leader, we just need to get a Preempted message that points to a leader we can't resolve an ActorRef for. Sending a Preempted with a bad actor path may do the trick.