debug mode fails if something is None

[kevinburke]

I'm trying to debug a pip test by passing debug=True to the script and getting the following output. It's not entirely clear what's going on because some of the stack trace is getting swallowed, but None is being passed to the string() function somewhere and this is failing, which sort of kills the debug mode functionality.

See for example https://travis-ci.org/kevinburke/pip/jobs/34864592#L229