invalid path to browser in conf/saunter.ini => inexplicable error message

[mmaypumphrey]

TO REPRODUCE:

1. Put a deliberately invalid path to the browser inside conf/saunter.ini:

browser: *chrome /Applications/Firefox-3.7.app/Contents/MacOS/firefox-bin

1. Run a known-to-work-all-the-time test:

adobe-MacBookPro:echosign mmaypump$ pysaunter.py -f logout
collected 20 items
F
========================================================= FAILURES =========================================================
/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium-2.6.0-py2.7.egg/selenium/selenium.py:224: TypeError: 'NoneType' object is not subscriptable
--------------- generated xml file: /Users/mmaypump/Desktop/qa/PYSAUNTER/echosign/logs/2011-09-23-48-09.xml ----------------
================================================ 19 tests deselected by '' =================================================
1 failed, 19 deselected in 0.62 seconds
adobe-MacBookPro:echosign mmaypump$

BUG: The average user needs a better error message than the TypeError one above for this particular scenario. In my case, I was attempting to bring up my pysaunter tests on a second system, which didn't have exactly the same path to the browser of interest.