send-pack/receive-pack: allow errors to be reported back to pusher.

send-pack/receive-pack: allow errors to be reported back to pusher.

This updates the protocol between git-send-pack/git-receive-pack
in a backward compatible way to allow failures at the receiving
end to be propagated back to the sender.  Most notably, versions
of git-push before this could not notice if the update hook on
the receiving end refused to update the ref for its own policy
reasons.

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