Extend Exception for custom error messages

app_exception.dart

// Create Exception classes by extending this class.
/// - Example:
/*
  class UnknownThemeMode extends AppException {
    UnknownThemeMode([String message, int code]) : super(message, 'Missing Context', code);
  }
*/

class AppException implements Exception {
  final message;
  final prefix;
  final code;
  AppException([this.message, this.prefix, this.code]);

  String toString() {
    return '{"$prefix" : $message", "code":$code}';
  }
}

Added a some instructions

better instruction layout