add an error message for cloning private repos

Show a nice error message when trying to clone a private repo:

![screen shot 2014-05-19 at 11 46 22 pm](https://cloud.githubusercontent.com/assets/1269969/3023759/99588798-dfec-11e3-9dac-ae877405159d.png)

Remove an older exception class in favor of the `SparkException` class.

@gaurave