SPARK-6389 YARN app diagnostics report doesn't report NPEs

Trivial patch to implicitly call `Exception.toString()` over `Exception.getMessage()` —this defaults to including the exception class & any non-null message; some subclasses include more.

No test.

Author: Steve Loughran <stevel@hortonworks.com>

Closes #5070 from steveloughran/stevel/patches/SPARK-6389-NPE-reporting and squashes the following commits:

8239d85 [Steve Loughran] SPARK-6389 cull use of getMessage over toString in the container launcher
6fbaf6a [Steve Loughran] SPARK-6389 YARN app diagnostics report doesn't report NPEs

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

@@ -151,7 +151,7 @@ private[spark] class ApplicationMaster(
         logError("Uncaught exception: ", e)
         finish(FinalApplicationStatus.FAILED,
           ApplicationMaster.EXIT_UNCAUGHT_EXCEPTION,
-          "Uncaught exception: " + e.getMessage())
+          "Uncaught exception: " + e)
     }
     exitCode
   }
@@ -486,10 +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)
+                logError("User class threw exception: " + cause, cause)
                 finish(FinalApplicationStatus.FAILED,
                   ApplicationMaster.EXIT_EXCEPTION_USER_CLASS,
-                  "User class threw exception: " + cause.getMessage)
+                  "User class threw exception: " + cause)
             }
         }
       }