pypa/pip — pull request #554 by msabramo (2012-06-01 22:14:18+00:00)
Title: Make the unknown VCS error message generate its text from the actual list of known VCSs rather than hard-coding the ones supported now.

The idea came for this while debugging this error occurring on Python 3.3 and realizing that vcs.backends was empty -- if the error message showed this it would've pointed me to the error much sooner.