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/object-api-errors.go

@@ -162,7 +162,7 @@ func toObjectErr(oerr error, params ...string) error {
 			apiErr.Type = v.Type
 		}
 		return apiErr
-	case errErasureWriteQuorum.Error():
+	case errErasureWriteQuorum.Error(), errCommitMetaMissing.Error():
 		apiErr := InsufficientWriteQuorum{}
 		if len(params) >= 1 {
 			apiErr.Bucket = params[0]