Fix missing link warning in gltf-model-plus.js

[HussainAther]

Summary

• Added a console.warn message in gltf-model-plus.js when a link is missing or improperly defined.
• Helps with debugging link-related issues and prevents silent failures.

Changes

• Added a check to verify ext.link and ext.link.href before processing.
• Outputs a console warning when a link is missing.

Testing

• Ensure media links load correctly.
• If a link is missing, check for console warnings in the browser console.

[DougReeder]

Looks like you'll need to cherry-pick these commits as well:
50d13ce
4f60634

I know this seems like a lot of hoops to jump through, but soon you'll have internalized how to make a clean pull request!

I always check the "files changed" tab before submitting a pull request, to ensure my pull request contains what I think it contains.