Add IBL CHANGES.md fix

[OmarShehata]

I forgot to add a CHANGES.md entry for my IBL error-reporting fix in #8303.

[cesium-concierge]

Thanks for the pull request @OmarShehata!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[ggetz]

Thanks @OmarShehata !