ninja win: Don't return unfound VS version, raise instead

Like r1978, but for ninja only.  The msvs generator can use the fallback, but
the ninja generator cannot do anything without knowing the path to the VS
installation.  It is better to fail here, than to fail further along when
trying to run the setup script.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/708303002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1999 78cadc50-ecff-11dd-a971-7dbc132099af