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

Fix run-example script to fail fast with useful error message if multiple
example assembly JARs are present.

Author: Venkata Ramana Gollamudi <ramana.gollamudi@huawei.com>

Closes #3377 from gvramana/run-example_fails and squashes the following commits:

fa7f481 [Venkata Ramana Gollamudi] Fixed review comments, avoiding ls output scanning.
6aa1ab7 [Venkata Ramana Gollamudi] Fix run-examples script error during multiple jars