def add(error)
      if @list.key? error.name
        raise DuplicateError, "An error named #{error.name} already exists"
      end
      @list[error.name] = error
    end
def add(error)
      if @list.key? error.name
        raise DuplicateError, "An error named #{error.name} already exists"
      end
      @list[error.name] = error
    end
Add a error to the list

 @param [Kanpachi::Error] error The error to add.
 @return [Hash<Kanpachi::Error>] All the added errors.
 @raise DuplicateError If a error is being duplicated.
 @api public