Error highlights stay out of sync during heavy use

When I use CodeWorld myself, the error highlights are pretty reliable, but during classes when many students are working at once, I often see students with out of date error highlights that won't update, even when they've stopped for a long time. I suspect that when the latency gets too long, something goes wrong in the client and errors get stuck out-of-sync. Maybe it loses track of which outstanding requests it should be cancelling, and cancels the latest one instead of earlier ones. Not sure.

In any case, when error highlights are too old, we should really consider removing them from the document, and perhaps displaying some kind of loading indicator to say we're working on updating the errors.