Raise error event for network link failure

[hpinkos]

Fixes #5921

We raise the error event in other places, but we missed this one.

[cesium-concierge]

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[hpinkos]

@thw0rted does this fix the problem you saw?

[thw0rted]

LGTM

[juburr]

@hpinkos Will this silently fail and continue loading the rest of the static content like here? #5871
If not, this would be desirable.

I've seen this happen with other resources as well (for example, I believe this can also happen with external icon image hrefs when not using a proxy). Ideally it should continue loading the rest of the KML.

[hpinkos]

@jburr-nc this change doesn't change anything about the way KML currently loads. The only difference is that the error event will be raised in the event of a network link failure. You would only see different behavior in your app if you are listening to the error event.

I thought we fixed the problem where a datasource wouldn't load when an image for a billboard failed. I couldn't find the pull request related to that though. If you have an example KML where that happens, let me know and I can write up a new issue.