[SPARK-6058][Yarn] Log the user class exception in ApplicationMaster

Because ApplicationMaster doesn't set SparkUncaughtExceptionHandler, the exception in the user class won't be logged. This PR added a `logError` for it.

Author: zsxwing <zsxwing@gmail.com>

Closes #4813 from zsxwing/SPARK-6058 and squashes the following commits:

806c932 [zsxwing] Log the user class exception

yarn/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala

@@ -486,11 +486,10 @@ private[spark] class ApplicationMaster(
               case _: InterruptedException =>
                 // Reporter thread can interrupt to stop user class
               case cause: Throwable =>
+                logError("User class threw exception: " + cause.getMessage, cause)
                 finish(FinalApplicationStatus.FAILED,
                   ApplicationMaster.EXIT_EXCEPTION_USER_CLASS,
                   "User class threw exception: " + cause.getMessage)
-                // re-throw to get it logged
-                throw cause
             }
         }
       }