cmdout/svk raise unhandled exception

[Andrew Beyer <beyer andrew gmail com>]

They were only a little bit ago, so this may be work in progress
awaiting another change, but the last couple commits on git break VC
on systems without svk as misc.cmdout called from svk's is_repo_root
raises an unhandled OSError when the svk executable isn't present
during vc enumeration.