Show an error message for dh.ui panels that could not rehydrate

[mattrunyon]

With the addition of widget re-hydration in dh.ui, there is currently no error message propagated to the panels if the widget fails to load. They are currently just blank. We should propagate the error message to the panels from the widget in this case as the panels still exist.

These steps should get you into the state where the error should be displayed

1. Ensure "Close panels on disconnect" is disabled
2. Create and render a dh.ui panel
3. Restart the server
4. Reload the web UI