Better KML Network Link Error Handling

[hpinkos]

In #5871 we added a catch when a network link fails. This makes it impossible to detect and handle errors programmatically. We might want to consider adding support for a callback when a network link fails.

[thw0rted]

It looks like failed network links already raise the errorEvent, which is good, except for this one overlooked case, which is an easy fix. Compare with L2427 -- basically, any loadBlob.otherwise() should be raising the error event and the linked PR didn't follow that pattern.

This bit me today. I loaded a file (from a "good" server) that includes a NetworkLink on a "bad" server, with a self-signed cert I hadn't manually added an exception for. The initial (KML) load was fine, so the error handler I'd attached to the promise returned by src.load() didn't fire, but the NetworkLink load (which of course fires asynchronously) didn't work. Anyway, easy one-line fix and I think this can be closed. I'd do it myself but I never got my dang management to sign off on the CLA. (I'll bug them again, I swear!)