Inelegant failure if extract script crashes or is killed

[fawkesley]

The view just shows "loading..." forever if the script fails (or runs out of memory...)

Need to write out and monitor a PID file and provide for a "failure" text in the view.

[frabcus]

This is less bad than it was because of the status endpoint.

It will at least recover next time the dataset changes.

Not sure how to do better than that without a cron job that runs all the time and checks for stale process.

[frabcus]

I think this is much better now