refs: add function to translate errors to strings

refs: add function to translate errors to strings

The commit 76e760b999 (refs: introduce enum-based transaction error
types, 2025-04-08) introduced enum-based transaction error types. The
refs transaction logic was also modified to propagate these errors. For
clients of the ref transaction system, it would be beneficial to provide
human readable messages for these errors.

There is already an existing mapping in 'builtin/update-ref.c', move it
to 'refs.c' as `ref_transaction_error_msg()` and use the same within the
'builtin/update-ref.c'.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>