Provide embedder feedback if openxr backend isn't set up right

[jdm]

Ideally we should pop up some kind of message dialog either on startup or when entering VR/AR fails. This requires some kind of coordination between the webxr backend, the DOM XR code, and the embedding API.

[jdm]

This is a bad enough experience that we should definitely fix it before publicizing our desktop UWP builds to a wider audience. It's really easy to forget to tell people to set up OpenXR first, and the experience of having a runtime that isn't enabled appears to be that xrCreateInstance returns XR_ERROR_INSTANCE_LOST.