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.
Instead of passing a second argument to `then`, you should use `.otherwise(getFailedLoadFunction(model, 'gltf', url));`  `otherwise` will catch all exceptions that may get generated while the second argument only catches exceptions in the first argument's function.