Runtime Errors when a tileset contains unsupported extensions

[srothst1]

Fixes Issue #9552
There is currently no _extensionsRequired property in Cesium3DTileset. This pull request uses the _extensions property.

Should we add _extensionsRequired?

cc @ptrgags @lilleyse

[cesium-concierge]

Thanks for the pull request @srothst1!

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• Works in 2D/CV.
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[lilleyse]

@srothst1 did you mean to merge this? Can you open a PR to revert this change? Then after that PR is merged you can re-open this PR and @ptrgags should have a chance to review it properly.

CC @shehzan10