refs: introduce enum-based transaction error types

Replace preprocessor-defined transaction errors with a strongly-typed
enum `ref_transaction_error`. This change:

  - Improves type safety and function signature clarity.
  - Makes error handling more explicit and discoverable.
  - Maintains existing error cases, while adding new error cases for
    common scenarios.

This refactoring paves the way for more comprehensive error handling
which we will utilize in the upcoming commits to add batch reference
update support.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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