implement error code enum

+/// This enum defines the different error codes for the mobile scanner.
+enum MobileScannerErrorCode {
+  /// The controller was used
+  /// while it was not yet initialized using [MobileScannerController.start].
+  controllerUnititialized,
+
+  /// A generic error occurred.
+  ///
+  /// This error code is used for all errors that do not have a specific error code.
+  genericError,
+
+  /// The permission to use the camera was denied.
+  permissionDenied,
+}