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

I'd prefer to avoid pattern matching on `Option`, so:
```scala
executorDataMap.get(executorId).map { executorInfo =>
  executorInfo.executorActor ! KillTask(taskId, executorId, interruptThread)
} getOrElse { 
  logWarning(s"Attempted to kill task $taskId for unknown executor $executorId.")
}
```