Merge branch 'hn/refs-errno-cleanup' into seen

Merge branch 'hn/refs-errno-cleanup' into seen

Futz with the way 'errno' is relied on in the refs API to carry the
failure modes up the callchain.

* hn/refs-errno-cleanup:
  refs: explicitly propagate errno from refs_read_raw_ref
  refs: clear errno return in refs_resolve_ref_unsafe()
  refs: add failure_errno to refs_read_raw_ref() signature
  refs: use refs_resolve_ref_unsafe_with_errno() where needed
  refs: make errno output explicit for refs_resolve_ref_unsafe
  refs: make errno output explicit for read_raw_ref_fn
  refs/files-backend: stop setting errno from lock_ref_oid_basic
  refs: remove EINVAL errno output from specification of read_raw_ref_fn