GIT-VERSION-GEN: squelch unneeded error from "cat version"

Now this is really a corner case, but if you have the git source
tree from somewhere other than the official tarball, you do not
have version file.  And if git-describe does not work for you
(maybe you do not have git yet), we spilled an error message
from "cat version".

Signed-off-by: Junio C Hamano <junkio@cox.net>