Merge pull request #1716 from jasongrout/error

Make error message more user-friendly.

Author: jasongrout

widgetsnbextension/src/extension.js

@@ -120,7 +120,7 @@ function register_events(Jupyter, events, outputarea) {
                 PhosphorWidget.Widget.attach(view.pWidget, node);
             });
         } else {
-            node.textContent = "Error rendering Jupyter widget. Widget not found: "+JSON.stringify(data);
+            node.textContent = `A Jupyter widget could not be displayed because the widget state could not be found. This could happen if the kernel storing the widget is no longer available, or if the widget state was not saved in the notebook. You may be able to create the widget by running the appropriate cells.`;
         }
     }