Don't reject the whole KML Data load on Network Link failure

[kiselev-dv]

GoogleEarth warns user when the link isn't available, but loads the rest of the KML.

This patch makes Cesium behave the same way, if NetworkLink promise will be rejected it will warn the user, but still loads the rest of the KML and resolves the whole KMLDataSource.load promise.

[cesium-concierge]

@kiselev-dv, thanks for the pull request! Maintainers, we have a signed CLA from @kiselev-dv, so you can review this at any time.

I noticed that CHANGES.md has not been updated. If this change updates the public API in any way, fixes a bug, or makes any non-trivial update, please add a bullet point to CHANGES.md and comment on this pull request so we know it was updated. For more info, see the Pull Request Guidelines.

I am a bot who helps you make Cesium awesome! Thanks again.

[ggetz]

Thanks for the PR @kiselev-dv !

For background, the rationale behind this is discussed more on the forum here: https://groups.google.com/forum/#!topic/cesium-dev/IdMz-Y6i7d4

[hpinkos]

Thanks @kiselev-dv!

[cesium-concierge]

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

https://groups.google.com/forum/#!topic/cesium-dev/IdMz-Y6i7d4

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

The issue at #5871 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.

I am a bot who helps you make Cesium awesome! Thanks again.

[hpinkos]

Updated CHANGES in c2f3bba