receive-pack: relay connectivity errors to sideband
If the connectivity check encounters a problem when
receiving a push, the error output goes to receive-pack's
stderr, whose destination depends on the protocol used
(ssh tends to send it to the user, though without a "remote"
prefix; http will generally eat it in the server's error
log).
The information should consistently go back to the user, as
there is a reasonable chance their client is buggy and
generating a bad pack.
We can do so by muxing it over the sideband as we do with
other sub-process stderr.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>