Update "unregistered_view_type" error message

Author: dacianf

lib/web_ui/lib/src/engine/platform_views/message_handler.dart

@@ -73,7 +73,8 @@ class PlatformViewMessageHandler {
     if (!_contentManager.knowsViewType(viewType)) {
       callback(_codec.encodeErrorEnvelope(
         code: 'unregistered_view_type',
-        message: 'trying to create a view with an unregistered type',
+        message: 'trying to create a view with an unregistered type. '
+            'Have you called GeneratedPluginRegistrant.register?',
         details: 'unregistered view type: $viewType',
       ));
       return;

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm

@@ -140,7 +140,8 @@
   NSObject<FlutterPlatformViewFactory>* factory = factories_[viewType].get();
   if (factory == nil) {
     result([FlutterError errorWithCode:@"unregistered_view_type"
-                               message:@"trying to create a view with an unregistered type"
+                               message:@"trying to create a view with an unregistered type. Have "
+                                       @"you called GeneratedPluginRegistrant.register?"
                                details:[NSString stringWithFormat:@"unregistered view type: '%@'",
                                                                   args[@"viewType"]]]);
     return;

shell/platform/darwin/macos/framework/Source/FlutterPlatformViewController.mm

@@ -39,7 +39,8 @@ - (void)onCreateWithViewID:(int64_t)viewId
   if (!factory) {
     result([FlutterError
         errorWithCode:@"unregistered_view_type"
-              message:@"trying to create a view with an unregistered type"
+              message:@"trying to create a view with an unregistered type. Have you called "
+                      @"GeneratedPluginRegistrant.register?"
               details:[NSString stringWithFormat:@"unregistered view type: '%@'", viewType]]);
     return;
   }