[SPARK-31632][CORE][WEBUI][FOLLOWUP] Enrich the exception message whe…

…n application summary is unavailable

### What changes were proposed in this pull request?
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This PR enriches the exception message when application summary is not available.
#28444 covers the case when application information is not available but the case application summary is not available is not covered.

### Why are the changes needed?
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To complement #28444 .

### Does this PR introduce _any_ user-facing change?
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Yes.
Before this change, we can get the following error message when we access to `/jobs` if application summary is not available.
<img width="707" alt="no-such-element-exception-error-message" src="https://user-images.githubusercontent.com/4736016/84562182-6aadf200-ad8d-11ea-8980-d63edde6fad6.png">

After this change, we can get the following error message. It's like #28444 does.
<img width="1349" alt="enriched-errorm-message" src="https://user-images.githubusercontent.com/4736016/84562189-85806680-ad8d-11ea-8346-4da2ec11df2b.png">

### How was this patch tested?
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I checked with the following procedure.
1. Set breakpoint in the line of `kvstore.write(appSummary)` in `AppStatusListener#onStartApplicatin`. Only the thread reaching this line should be suspended.
2. Start spark-shell and wait few seconds.
3. Access to `/jobs`

Closes #28820 from sarutak/fix-no-such-element.

Authored-by: Kousuke Saruta <sarutak@oss.nttdata.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
(cherry picked from commit c2e5012)
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

core/src/main/scala/org/apache/spark/status/AppStatusStore.scala

@@ -519,7 +519,13 @@ private[spark] class AppStatusStore(
   }
 
   def appSummary(): AppSummary = {
-    store.read(classOf[AppSummary], classOf[AppSummary].getName())
+    try {
+      store.read(classOf[AppSummary], classOf[AppSummary].getName())
+    } catch {
+      case _: NoSuchElementException =>
+        throw new NoSuchElementException("Failed to get the application summary. " +
+          "If you are starting up Spark, please wait a while until it's ready.")
+    }
   }
 
   def close(): Unit = {