The description of error_message_format does not give enough information to use it.  The description should talk about how it is used (ie: using % formatting) and the fact that when filled in it should form a complete html document because it will be sent as the error response body.  It should not talk about what "the code key should be", etc, but should instead say what they *will* be, and cross reference the send_error method as the thing that uses it.