IGWN Alert listener fail without triggering nagios checks
We've had the igwn alert listener fail several times in the last week (see #441 (closed)) and nagios never notices, but GWCelery essentially stops functioning in that it no longer does anything it'd do in response to an IGWN-Alert. Although it currently seems like these issues are upstream of us (possibly in the broker), we need to come up with a nagios check we can add for this so that we don't go hours to days at a time without realizing new superevents haven't been created. Right now we've been relying on gracedb-playground users telling us that no new superevents are showing up, which is obviously not a good plan for observing runs.