Catch and report error when IBL map fails to load

[OmarShehata]

This came up on the forum. If you take the old_venetian_crossroads_2k_ibl in this zip, and try to load it in the IBL Sandcastle it fails to load, and the only indication you get is that the models are just completely black.

EnvironmentMap.zip

I eventually realized CesiumJS was failing to load that KTX file because it had an unsupported glInternalFormat of R11F_G11F_B10F. This PR gets OctahedralProjectedCubeMap to reject its readyPromise when it fails to load the environment map, and gets Model.js to listen on that and console.error it out. I also added a test to make sure the promise is rejected when it tries to load an invalid URL.

This KTX file was generated from cmgen 1.4.1, but it seems to have this unsupported format because of some recent change. I tested with cmgen 1.2.0 and it works in CesiumJS (included in the above zip).

[cesium-concierge]

Thanks for the pull request @OmarShehata!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

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

[cesium-concierge]

Thanks again for your contribution @OmarShehata!

No one has commented on this pull request in 30 days. Maintainers, can you review, merge or close to keep things tidy?

I'm going to re-bump this in 30 days. If you'd like me to stop, just comment with @cesium-concierge stop. If you want me to start again, just delete the comment.

[cesium-concierge]

Congratulations on closing the issue! I found these Cesium forum links in the comments above:

https://groups.google.com/d/msg/cesium-dev/6C8O1faCEFc/CvcK83ZyDgAJ

If this issue affects any of these threads, please post a comment like the following:

The issue at #8303 has just been closed and may resolve your issue. Look for the change in the next stable release of Cesium or get it now in the master branch on GitHub https://github.com/AnalyticalGraphicsInc/cesium.

[OmarShehata]

Thanks @emackey !