replace error message with error code

+import 'package:mobile_scanner/src/enums/mobile_scanner_error_code.dart';
+
+/// This class represents an exception throzn by the mobile scanner.
 class MobileScannerException implements Exception {
 class MobileScannerException implements Exception {
-  String message;
-  MobileScannerException(this.message);
+  MobileScannerException({required this.errorCode});
+
+  /// The error code of the exception.
+  final MobileScannerErrorCode errorCode;
 }
 }