[SPARK-7534] [CORE] [WEBUI] Fix the Stage table when a stage is missing

Just improved the Stage table when a stage is missing.

Before:

![screen shot 2015-05-11 at 10 11 51 am](https://cloud.githubusercontent.com/assets/1000778/7570842/2ba37380-f7c8-11e4-9b5f-cf1a6264b2a4.png)

After:

![screen shot 2015-05-11 at 10 26 09 am](https://cloud.githubusercontent.com/assets/1000778/7570848/33703152-f7c8-11e4-81a8-d53dd72d7b8d.png)

Author: zsxwing <zsxwing@gmail.com>

Closes #6061 from zsxwing/SPARK-7534 and squashes the following commits:

09fe862 [zsxwing] Leave it blank rather than '-'
6299197 [zsxwing] Fix the Stage table when a stage is missing

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

@@ -121,10 +121,23 @@ private[ui] class StageTableBase(
     <div>{stageDesc.getOrElse("")} {killLink} {nameLink} {details}</div>
   }
 
+  protected def missingStageRow(stageId: Int): Seq[Node] = {
+    <td>{stageId}</td> ++
+    {if (isFairScheduler) {<td>-</td>} else Seq.empty} ++
+    <td>No data available for this stage</td> ++ // Description
+    <td></td> ++ // Submitted
+    <td></td> ++ // Duration
+    <td></td> ++ // Tasks: Succeeded/Total
+    <td></td> ++ // Input
+    <td></td> ++ // Output
+    <td></td> ++ // Shuffle Read
+    <td></td> // Shuffle Write
+  }
+
   protected def stageRow(s: StageInfo): Seq[Node] = {
     val stageDataOption = listener.stageIdToData.get((s.stageId, s.attemptId))
     if (stageDataOption.isEmpty) {
-      return <td>{s.stageId}</td><td>No data available for this stage</td>
+      return missingStageRow(s.stageId)
     }
 
     val stageData = stageDataOption.get