=str #16924: Fix FanoutProcessor to not overwrite last termination cause

Also fix FlowSpec to expect onError instead of onComplete for late subscribers

akka-stream/src/main/scala/akka/stream/impl/SubscriberManagement.scala

@@ -32,7 +32,7 @@ private[akka] object SubscriberManagement {
     def apply[T](subscriber: Subscriber[T]): Unit = tryOnError(subscriber, cause)
   }
 
-  val ShutDown = new ErrorCompleted(new IllegalStateException("Cannot subscribe to shut-down Publisher"))
+  val ShutDown = new ErrorCompleted(ActorPublisher.NormalShutdownReason)
 }
 
 /**