Proxy ErrorSet.default_error_messages= to MessageTransformer::Default.error_messages=.
Proxy ErrorSet.default_error_messages= to MessageTransformer::Default.error_messages=.

I forgot to preserve this API (I did preserve
ErrorSet.default_error_message, but forgot the setter), and dm-devise
relies on it.

mbj/aequitas,emmanuel/aequitas,datamapper/dm-validations