SPARK-1933: Throw a more meaningful exception when a directory is passed to addJar/addFile.

https://issues.apache.org/jira/browse/SPARK-1933

Author: Reynold Xin <rxin@apache.org>

Closes #888 from rxin/addfile and squashes the following commits:

8c402a3 [Reynold Xin] Updated comment.
ff6c162 [Reynold Xin] SPARK-1933: Throw a more meaningful exception when a directory is passed to addJar/addFile.