[SPARK-4504][Examples] fix run-example failure if multiple example jars present

Fix run-examples script error during multiple jars.
Modified to support failing fast similar to assembly.jar

Author: Venkata Ramana G <ramana.gollamudi@huawei.com>
Well, I guess the inconsistency was there before, too, since the "you need to build Spark before running this program" has it.