An exception template which you can extend (inheritance... since `Exceptions` typically have the same implementations)

customizable_exception.dart

import 'package:meta/meta.dart';
// import 'package:flutter/foundation.dart'; // Alternative for Flutter

@immutable
abstract class CustomizableException implements Exception {
  static const String blankMsg = '';
  static const String divider = ': ';
  
  final String msg;
  
  const CustomizableException([msg]): msg = msg ?? blankMsg;

  @override
  String toString() => '$runtimeType$divider$msg';
}

@immutable
class CustomException extends CustomizableException {
  const CustomException([msg]): super(msg);
}