clone: correctly report http_fetch errors

clone: correctly report http_fetch errors

The exit status from curl was accidentally lost by the
'case' statement. We need to explicitly save it so that $?
doesn't get overwritten.

This improves the error message when fetching from an http
repository which has never had update-server-info run.
Previously, it would fail to note the fetch error and
produce multiple errors about the lack of origin branches.
It now correctly suggests running git-update-server-info.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>