Catches BadCommand in get_src_requirement()

This catches the BadCommand that get raised by vcs modules when the VCS
software is not installed or not in the PATH. In this case, we behave as
if the .git, .svn or .hg folder hadn't been found at all (but with the
correct warning).
Fixes pep8 issues
Merge pull request #1830 from remram44/vcs-not-found_2

Catches BadCommand in get_src_requirement()