Whenever you specify a `code`, please use a descriptive symbol, e.g. `:username_missing`.  Then you can give a separate `message`.  See https://github.com/openstax/exercises/blob/master/app/handlers/terms_agree.rb#L23 for an example.  

`code` is intended to be something used in test code (for easily testing bad outcomes) and eventually in locale files when we do i18n stuff.