parse code from platform exception; use shorter message; use if-case to extract size

@@ -187,8 +187,7 @@ class MethodChannelMobileScanner extends MobileScannerPlatform {
       throw const MobileScannerException(
         errorCode: MobileScannerErrorCode.controllerAlreadyInitialized,
         errorDetails: MobileScannerErrorDetails(
-          message:
-              'The scanner was already started. Call stop() before calling start() again.',
+          message: 'The scanner was already started.',
         ),
       );
     }
@@ -204,7 +203,7 @@ class MethodChannelMobileScanner extends MobileScannerPlatform {
       );
     } on PlatformException catch (error) {
       throw MobileScannerException(
-        errorCode: MobileScannerErrorCode.genericError,
+        errorCode: MobileScannerErrorCode.fromPlatformException(error),
         errorDetails: MobileScannerErrorDetails(
           code: error.code,
           details: error.details as Object?,
@@ -240,17 +239,13 @@ class MethodChannelMobileScanner extends MobileScannerPlatform {
       startResult['currentTorchState'] as int? ?? -1,
     );
 
-    final Map<Object?, Object?>? sizeInfo =
-        startResult['size'] as Map<Object?, Object?>?;
-    final double? width = sizeInfo?['width'] as double?;
-    final double? height = sizeInfo?['height'] as double?;
-
     final Size size;
 
-    if (width == null || height == null) {
-      size = Size.zero;
-    } else {
+    if (startResult['size']
+        case {'width': final double width, 'height': final double height}) {
       size = Size(width, height);
+    } else {
+      size = Size.zero;
     }
 
     return MobileScannerViewAttributes(