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/storage-rest-client.go

@@ -106,6 +106,8 @@ func toStorageErr(err error) error {
 		return errVolumeExists
 	case errFileNotFound.Error():
 		return errFileNotFound
+	case errCommitMetaMissing.Error():
+		return errCommitMetaMissing
 	case errFileVersionNotFound.Error():
 		return errFileVersionNotFound
 	case errFileNameTooLong.Error():