[SPARK-12399] Display correct error message when accessing REST API with an unknown app Id

I got an exception when accessing the below REST API with an unknown application Id.
`http://<server-url>:18080/api/v1/applications/xxx/jobs`
Instead of an exception, I expect an error message "no such app: xxx" which is a similar error message when I access `/api/v1/applications/xxx`
```
org.spark-project.guava.util.concurrent.UncheckedExecutionException: java.util.NoSuchElementException: no app with key xxx
	at org.spark-project.guava.cache.LocalCache$Segment.get(LocalCache.java:2263)
	at org.spark-project.guava.cache.LocalCache.get(LocalCache.java:4000)
	at org.spark-project.guava.cache.LocalCache.getOrLoad(LocalCache.java:4004)
	at org.spark-project.guava.cache.LocalCache$LocalLoadingCache.get(LocalCache.java:4874)
	at org.apache.spark.deploy.history.HistoryServer.getSparkUI(HistoryServer.scala:116)
	at org.apache.spark.status.api.v1.UIRoot$class.withSparkUI(ApiRootResource.scala:226)
	at org.apache.spark.deploy.history.HistoryServer.withSparkUI(HistoryServer.scala:46)
	at org.apache.spark.status.api.v1.ApiRootResource.getJobs(ApiRootResource.scala:66)
```

Author: Carson Wang <carson.wang@intel.com>

Closes #10352 from carsonwang/unknownAppFix.