Model viewer app

I want to use this in an iframe for previewing glTF models on the website.

I also made a tiny change to `Model.fromGltf` so it throws a runtime error when it can't load the file at the given URL.
@shunter can verify this, but I think yes; use of `otherwise` should be Cesium-wide (though just changing `Model` is fine for now) I think most other places are already using `otherwise`