Inform user about expired sample tokens

[javagl]

The ion tokens that are distributed with the samples are usually valid for two months. With the monthly release, there is some grace period for people to have a chance to update, and receive new tokens. But when the tokens expire, the "end user experience" might just be that ~"nothing appears, everything is broken" (it just prints some error message on the console that doesn't tell much about the reason).

Users should be informed about the reason why "nothing appears", in a way that is easy to understand and accessible. We could go fancy there, and present that error to the user, maybe as a dialog with some link to the site for updating the samples (and of course, for registering on ion, so that fewer people rely on the sample tokens).

This affects all levels:

• In cesium-native, there should be a mechanism for more detailed error messages. The point for starting to address this may be near Tileset::_handleAssetResponse, where we might check the return code (401 for "Unauthorized", I think) and somehow retain this information. This could be along the lines of some enum TilesetError (not using the HTTP codes, because we might need some PARSING_ERROR error types as well), and a Tileset::getLastError or so
• In cesium-unreal, there should be a method for obtaining that information and making it available via the Cesium3DTileset (accessible via Blueprints and such)
• In cesium-unreal-samples there might be a dedicated blueprint to handle the case of expired tokens.

The latter could make sense, because in the usual workflow, a 401 error would roughly mean that the user misconfigured his ionToken. A dedicated blueprint for the samples could capture this error, and identify it as the special case of "The sample token is expired" (because in the samples, there should be no other 401 errors).

All this is related to (and largely depends on) CesiumGS/cesium-unreal#542