refs: explicitly propagate errno from refs_read_raw_ref

refs: explicitly propagate errno from refs_read_raw_ref

The function refs_resolve_ref_unsafe_with_errno should produce an errno output.
Rather than taking the value from the errno (which might contain garbage
beforehand), explicitly propagate the failure_errno coming out of
refs_read_raw_ref().

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>