WSGI: grab DXR_FOLDER from os.environ if not specified in WSGI environ

This makes it easier to pass it in in some cases (e.g. stackato)

This should properly die if it's not given anywhere.  Also, updated the comments, and tried to be more verbose.  I guess blindly-leap-ahead-and-catch-the-exception is more Pythonic anyway...

Not sure why github decided to just... disappear my previous pull request.  Perhap because I rebased + amended?