[Test] Better exception message from SparkSubmitSuite

Before:
```
Exception in thread "main" java.lang.Exception: Could not load user defined classes inside of executors
	at org.apache.spark.deploy.JarCreationTest$.main(SparkSubmitSuite.scala:471)
	at org.apache.spark.deploy.JarCreationTest.main(SparkSubmitSuite.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
```
After:
```
Exception in thread "main" java.lang.Exception: Could not load user class from jar:
java.lang.UnsupportedClassVersionError: SparkSubmitClassA : Unsupported major.minor version 51.0
	java.lang.ClassLoader.defineClass1(Native Method)
	java.lang.ClassLoader.defineClass(ClassLoader.java:643)
	...
	at org.apache.spark.deploy.JarCreationTest$.main(SparkSubmitSuite.scala:472)
	at org.apache.spark.deploy.JarCreationTest.main(SparkSubmitSuite.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
```

Author: Andrew Or <andrew@databricks.com>

Closes #3212 from andrewor14/submit-suite-message and squashes the following commits:

7779248 [Andrew Or] Format exception
8fe6719 [Andrew Or] Better exception message from failed test

core/src/test/scala/org/apache/spark/deploy/SparkSubmitSuite.scala

@@ -21,7 +21,7 @@ import java.io._
 
 import scala.collection.mutable.ArrayBuffer
 
-import org.apache.spark.{SparkConf, SparkContext, SparkEnv, SparkException, TestUtils}
+import org.apache.spark._
 import org.apache.spark.deploy.SparkSubmit._
 import org.apache.spark.util.Utils
 import org.scalatest.FunSuite
@@ -451,24 +451,25 @@ class SparkSubmitSuite extends FunSuite with Matchers {
   }
 }
 
-object JarCreationTest {
+object JarCreationTest extends Logging {
   def main(args: Array[String]) {
     Utils.configTestLog4j("INFO")
     val conf = new SparkConf()
     val sc = new SparkContext(conf)
     val result = sc.makeRDD(1 to 100, 10).mapPartitions { x =>
-      var foundClasses = false
+      var exception: String = null
       try {
         Class.forName("SparkSubmitClassA", true, Thread.currentThread().getContextClassLoader)
         Class.forName("SparkSubmitClassA", true, Thread.currentThread().getContextClassLoader)
-        foundClasses = true
       } catch {
-        case _: Throwable => // catch all
+        case t: Throwable =>
+          exception = t + "\n" + t.getStackTraceString
+          exception = exception.replaceAll("\n", "\n\t")
       }
-      Seq(foundClasses).iterator
+      Option(exception).toSeq.iterator
     }.collect()
-    if (result.contains(false)) {
-      throw new Exception("Could not load user defined classes inside of executors")
+    if (result.nonEmpty) {
+      throw new Exception("Could not load user class from jar:\n" + result(0))
     }
   }
 }