receive-pack: utilize rejected ref error details

receive-pack: utilize rejected ref error details

In 9d2962a7c4 (receive-pack: use batched reference updates, 2025-05-19),
git-receive-pack(1) switched to using batched reference updates. This also
introduced a regression wherein instead of providing detailed error
messages for failed referenced updates, the users were provided generic
error messages based on the error type.

Now that the updates also contain detailed error message, propagate
those to the client via 'rp_error'. The detailed error messages can be
very verbose, for e.g. in the files backend, when trying to write a
non-commit object to a branch, you would see:

   ! [remote rejected] 3eaec9ccf3a53f168362a6b3fdeb73426fb9813d ->
   branch (cannot update ref 'refs/heads/branch': trying to write
   non-commit object 3eaec9ccf3a53f168362a6b3fdeb73426fb9813d to branch
   'refs/heads/branch')

Here the refname is repeated multiple times due to how error messages
are propagated and filled over the code stack. This potentially can be
cleaned up in a future commit.

Reported-by: Elijah Newren <newren@gmail.com>
Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>