toString method
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 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.
Implementation
@override
String toString({Object? color}) {
if (span == null) return message;
return 'Error on ${span!.message(message, color: color)}';
}