fetch: utilize rejected ref error details

fetch: utilize rejected ref error details

In 0e358de64a (fetch: use batched reference updates, 2025-05-19),
git-fetch(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.

Similar to the previous commit, switch to using detailed error messages
if present for failed reference updates to fix this regression.

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