Don't return unfound VS version, raise instead

This fallback behaviour may have worked at some point, but doesn't
any more, so simply error out with a clear message rather than trying
to continue.

R=timurrrr@chromium.org
BUG=https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/O_OGfq2nhSw

Review URL: https://codereview.chromium.org/595543003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1978 78cadc50-ecff-11dd-a971-7dbc132099af
1 file changed