code: func (m *MediaTypeDefinition) IsError() bool {
  	base, params, err := mime.ParseMediaType(m.Identifier)
  	if err != nil {
  		panic("invalid media type identifier " + m.Identifier) // bug
  	}
  	delete(params, "view")
  	return mime.FormatMediaType(base, params) == ErrorMedia.Identifier
  }
docs: // IsError returns true if the media type is implemented via a goa struct.