Terrain server check

Fixes #317.

Merge #405 first
Setting the `terrainProvider` property like this will cause Cesium to invalidate the whole terrain surface (showing just the stars again) and then rebuild it.  So I have to imagine this is going to increase startup time, at least, and possibly cause the globe to flash briefly.

I think it's better to assume success (that is, install the real terrain provider), and then just handle unexpected failure in a reasonable way.