push: finishing touches to explain REJECT_ALREADY_EXISTS better

Now that "already exists" errors are given only when a push tries to
update an existing ref in refs/tags/ hierarchy, we can say "the
tag", instead of "the destination reference", and that is far easier
to understand.

Pointed out by Chris Rorvick.

Signed-off-by: Junio C Hamano <gitster@pobox.com>