I think we can leave it as it is. If the deprecated methods returned a transformer that doesn't implement `MTLTransformerErrorHandling`, we'd take the other branch [here and in all similar cases](https://github.com/github/Mantle/blob/3054d9f9dffe7b89fb6fb43cbaee3aba0ed45156/Mantle/MTLJSONAdapter.m#L119) but the behavior would be the same and we'd still be treating `nil` as success.