refs: return `ref_transaction_error` from `ref_transaction_update()`

refs: return `ref_transaction_error` from `ref_transaction_update()`

The `ref_transaction_update()` function is used to add updates to a
given reference transactions. In the following commit, we'll add more
validation to this function. As such, it would be beneficial if the
function returns specific error types, so callers can differentiate
between different errors.

To facilitate this, return `enum ref_transaction_error` from the
function and covert the existing '-1' returns to
'REF_TRANSACTION_ERROR_GENERIC'. Since this retains the existing
behavior, no changes are made to any of the callers but this sets the
necessary infrastructure for introduction of other errors.

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