[BRANCH-1.1][SPARK-4626] Kill a task only if the executorId is (still) registered with the scheduler

v1.1 backport for #3483

Author: roxchkplusony <[email protected]>

Closes #3503 from roxchkplusony/bugfix/4626-1.1 and squashes the following commits:

234d350 [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 +118,12 @@ class CoarseGrainedSchedulerBackend(scheduler: TaskSchedulerImpl, actorSystem: A
         makeOffers()
 
       case KillTask(taskId, executorId, interruptThread) =>
-        executorActor(executorId) ! KillTask(taskId, executorId, interruptThread)
+        if (executorActor.contains(executorId)) {
+          executorActor(executorId) ! KillTask(taskId, executorId, interruptThread)
+        } else {
+          // 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