[SPARK-4626] Kill a task only if the executorId is (still) registered with the scheduler

Author: roxchkplusony <roxchkplusony@gmail.com>

Closes #3483 from roxchkplusony/bugfix/4626 and squashes the following commits:

aba9184 [roxchkplusony] replace warning message per review
5e7fdea [roxchkplusony] [SPARK-4626] Kill a task only if the executorId is (still) registered with the scheduler

Author: roxchkplusony

core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala

@@ -118,7 +118,13 @@ class CoarseGrainedSchedulerBackend(scheduler: TaskSchedulerImpl, actorSystem: A
         makeOffers()
 
       case KillTask(taskId, executorId, interruptThread) =>
-        executorActor(executorId) ! KillTask(taskId, executorId, interruptThread)
+        executorActor.get(executorId) match {
+          case Some(executorInfo) =>
+            executorInfo.executorActor ! KillTask(taskId, executorId, interruptThread)
+          case None =>
+            // Ignoring the task kill since the executor is not registered.
+            logWarning(s"Attempted to kill task $taskId for unknown executor $executorId.")
+        }
 
       case StopDriver =>
         sender ! true