deletion of dir from whence server was started => "'NoneType' object is not subscriptable"

[mmaypumphrey]

Okay, granted this one involves an even "dumber user" than in issue 8. However, I still think it would be super if the harness could give an improved error message, maybe one that lists all the known possible causes of this TypeError?

TO REPRODUCE:

1. Start the server up from a specific directory.
2. Remove that directory.
3. Run a test under pysaunter.
4. Get the less than helpful message:

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium-2.13.1-py2.7.egg/selenium/selenium.py:221: TypeError: 'NoneType' object is not subscriptable