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

How about 
```scala


        val executorInfoOpt = executorDataMap.get(executorId)
        if (executorInfoOpt.isDefined) {
          executorInfoOpt.get.executorActor ! 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.")
        }
```
:)