fix: return 503 instead of 404 when commit metadata is missing

When the staged xl.meta disappears between the data rename and the
meta-file commit inside RenameData(), the affected drives return
errFileNotFound. An in-quorum errFileNotFound is reduced by
reduceWriteQuorumErrs() and previously surfaced as:

- IncompleteBody (400) from PutObject(), misattributing the failure
  to a client disconnect, or
- ObjectNotFound (404 NoSuchKey) from CompleteMultipartUpload(),
  telling clients the object is absent so they skip retries.

Map errFileNotFound at the commit step to a dedicated internal
errCommitMetaMissing, convert it to errErasureWriteQuorum in
renameData(), and map it to InsufficientWriteQuorum (HTTP 503) in
toObjectErr() so clients retry. Below-quorum outcomes already return
InsufficientWriteQuorum and are unchanged, as is the IncompleteBody
mapping for staging failures of client-disconnected uploads.

Signed-off-by: Julien Laurenceau <julien.laurenceau@pepitedata.com>

cmd/erasure-object.go

@@ -1072,6 +1072,12 @@ func renameData(ctx context.Context, disks []StorageAPI, srcBucket, srcEntry str
 	errs := g.Wait()
 
 	err := reduceWriteQuorumErrs(ctx, errs, objectOpIgnoredErrs, writeQuorum)
+	if errors.Is(err, errCommitMetaMissing) {
+		// Commit metadata went missing on a quorum of drives; the write
+		// did not commit. Return a 503 class error so clients retry
+		// instead of treating the object as absent (404).
+		err = errErasureWriteQuorum
+	}
 	if err != nil {
 		dg := errgroup.WithNErrs(len(disks))
 		for index, nerr := range errs {