[SPARK-19807][WEB UI] Add reason for cancellation when a stage is killed using web UI

## What changes were proposed in this pull request?

When a user kills a stage using web UI (in Stages page), StagesTab.handleKillRequest requests SparkContext to cancel the stage without giving a reason. SparkContext has cancelStage(stageId: Int, reason: String) that Spark could use to pass the information for monitoring/debugging purposes.

## How was this patch tested?

manual tests

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: shaolinliu <liu.shaolin1@zte.com.cn>
Author: lvdongr <lv.dongdong@zte.com.cn>

Closes #17258 from shaolinliu/SPARK-19807.

core/src/main/scala/org/apache/spark/ui/jobs/StagesTab.scala

@@ -42,7 +42,7 @@ private[ui] class StagesTab(parent: SparkUI) extends SparkUITab(parent, "stages"
       val stageId = Option(request.getParameter("id")).map(_.toInt)
       stageId.foreach { id =>
         if (progressListener.activeStages.contains(id)) {
-          sc.foreach(_.cancelStage(id))
+          sc.foreach(_.cancelStage(id, "killed via the Web UI"))
           // Do a quick pause here to give Spark time to kill the stage so it shows up as
           // killed after the refresh. Note that this will block the serving thread so the
           // time should be limited in duration.