Add a callback for tileset and tile errors at runtime

[kring]

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

[j9liu]

~Just to clarify, is this asking for something different from the OnCesium3DTilesetLoadFailure event (and its overlay counterpart)? The event allows delegates to interact with Cesium3DTilesetLoadFailureDetails, which report the type of load failure and the http status code.~

Never mind, I didn't read closely :) so this should account for errors actually loading / parsing the data, instead of just whether or not it could be loaded from a resource.