map - handling missing terrain tiles from server more gracefully

[anselm]

In some cases terrain tiles are missing / are not returned. It's not clear what's always going on. Around San Francisco for example there are many missing tiles - this could be due to the pacific ocean which may not have terrain tiles?

In some areas this occurs as low as LOD level 10. In other areas terrain tiles exist down to 16. It would be nice to fill the missing tile segments with at least a vector square or ideally an ellipsoid hull segment.

https://cesiumjs.org/data-and-assets/terrain/stk-world-terrain/

https://cesiumjs.org/Cesium/Build/Documentation/CesiumTerrainProvider.html