I've implemented std::error, meeting the requirements laid out in [P1028], and based on this experience, have written up a paper outlining various design decisions and concerns that came up during the implementation, particularly with regard to how std::error might map to existing error mechanisms (dynamic exceptions, std::error_code).  

This is not a formal proposal right now.  It is intended simply to further discussion around the design, requirements and implementation details of some future std::error specification.