feat(runtime): Add Image node output "On Error" signal (#48)

"On Error" is triggered when the image is not loading correctly, making it possible to hide the Image node or show a placeholder instead.

packages/noodl-viewer-react/src/nodes/visual/image.js

@@ -125,6 +125,12 @@ const ImageNode = {
       propPath: 'dom',
       type: 'signal',
       group: 'Events'
+    },
+    onError: {
+      displayName: 'On Error',
+      propPath: 'dom',
+      type: 'signal',
+      group: 'Events'
     }
   }
 };