As I reported in #10, error keys get duplicated and we should namespace them. This code behaves right like AR::Base accept_nested_attributes

Using as * adds a module to the root namespace, so no prefix is required, but those members are still locally scoped to the current document.

Internal Sass features have also moved into the module system, so we have complete control over the global namespace.

Authors of third-party tools should prefix each label key with the reverse DNS notation of a domain they own, such as com.example.some-label.