Show an error panel when a render loop error occurs. 

Previously, a default `CesiumWidget` or `Viewer` would silently "freeze" if an error occurred, because by default nothing listens to the `onRenderLoopError` event.  

Also use the same error panel to indicate when other errors occur in CesiumViewer.

An easy way to test this until #1052 gets fixed is to use the `Base Materials > Fresnel` option in the Materials Sandcastle example, or by running the CesiumViewer app and dropping random junk into the window or loading junk via a query parameter.