Add a callback for tileset and tile errors at runtime

Unity counterpart of CesiumGS/cesium-unreal#542.

There should be a way to programmatically receive errors and warnings about tilesets and overlays at runtime. For example, if a URL is invalid, a message is printed to the log, but it's difficult or impossible to learn of this in a script and handle it in a useful way.

Recently mentioned on the forum here:
https://community.cesium.com/t/legacy-3dtiles-version/22962/3