Is called when a message isn't handled by the current behavior of the actor
by default it fails with either a DeathPactException (in
case of an unhandled Terminated message) or publishes an UnhandledMessage
to the actor's system's EventStream
Namespace:Akka.Actor Assembly:
Akka (in Akka.dll) Version: 1.2.3.129 (1.2.3.129)