use well defined error code for barcode errors on iOS

@@ -42,7 +42,10 @@ public class MobileScannerPlugin: NSObject, FlutterPlugin {
     init(barcodeHandler: BarcodeHandler, registry: FlutterTextureRegistry) {
         self.mobileScanner = MobileScanner(registry: registry, mobileScannerCallback: { barcodes, error, image in
             if error != nil {
-                barcodeHandler.publishEvent(["name": "error", "data": error!.localizedDescription])
+                barcodeHandler.publishEvent([
+                    "name": MobileScannerErrorCodes.BARCODE_ERROR,
+                    "data": error!.localizedDescription,
+                ])                
                 return
             }