Silence confusing and false-positive curl error message

Silence confusing and false-positive curl error message

git-http-fetch spits out curl 404 error message when unable to fetch an object,
but that's confusing since no error really happened and the object is usually
found in a pack it tries right after that. And if the object still cannot be
retrieved, it will say another error message anyway. OTOH other HTTP errors
(403 etc) are likely fatal and the user should be still informed about them.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>