toString method
override
    Returns a string representation of this.
color may either be a String, a bool, or null. If it's a string,
it indicates an ANSI terminal color escape that should be used to
highlight the primary span's text. If it's true, it indicates that the
text should be highlighted using the default color. If it's false or
null, it indicates that the text shouldn't be highlighted.
If color is true or a string, secondaryColor is used to highlight
secondarySpans.
Implementation
@override
String toString({Object? color, String? secondaryColor}) {
  if (span == null) return message;
  var useColor = false;
  String? primaryColor;
  if (color is String) {
    useColor = true;
    primaryColor = color;
  } else if (color == true) {
    useColor = true;
  }
  final formatted = span!.messageMultiple(
      message, primaryLabel, secondarySpans,
      color: useColor,
      primaryColor: primaryColor,
      secondaryColor: secondaryColor);
  return 'Error on $formatted';
}