OpenXR generates errors when attempting to enter immersive vr when no headset is plugged in

[chrisfromwork]

Repro:

1. Run a UWP experience
2. Attempt to launch into xr when no headset is plugged in

Outcome
Failure to launch into xr crashes the application

Expected Outcome:
Failure to launch into xr doesn't crash the application

[bghgary]

Closing for now. Reopen if we want to support this.