[SPARK-12663][MLLIB] More informative error message in MLUtils.loadLibSVMFile

This PR contains 1 commit which resolves [SPARK-12663](https://issues.apache.org/jira/browse/SPARK-12663).

For the record, I got a positive response from 2 people when I floated this idea on devspark.apache.org on 2015-10-23. [Link to archived discussion.](http://apache-spark-developers-list.1001551.n3.nabble.com/slightly-more-informative-error-message-in-MLUtils-loadLibSVMFile-td14764.html)

Author: Robert Dodier <robert_dodier@users.sourceforge.net>

Closes #10611 from robert-dodier/loadlibsvmfile-error-msg-branch.