Don't return unfound VS version, raise instead

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

Committed: https://code.google.com/p/gyp/source/detail?r=1978

[Timur Iskhodzhanov, 2014-09-23 14:02:46]

lgtm

[scottmg, 2014-09-23 16:20:21]

The CQ bit was checked by scottmg@chromium.org

[scottmg, 2014-09-23 16:20:24]

The CQ bit was unchecked by scottmg@chromium.org

[scottmg, 2014-09-23 16:21:31]

Committed patchset #1 (id:1) manually as 1978 (presubmit successful).