[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)
```
Hi @andrewor14 , only show one class not found? Is it better to show all classes not found when there are more than one?