Provide feedback when requesting an XR session fails during device setup

[jdm]

#26312 makes us check whether OpenXR initializes correctly at startup and displays a dialog if it doesn't. It would arguably be a nicer experience if you could browse normally and only saw the dialog when you tried to enter immersive mode.

This would require:

• exposing a particular new error result from webxr-api which can be recognized in xrsystem.rs
• recognizing that error and sending a message to the embedder to display a prompt
• returning that new error from the appropriate code in webxr when specific openxr error codes are returned when creating an instance or enumerating extensions