Add a snitch to the DB export script

[stevejalim]

Recently - eg #15220 - we've had times where the DB exporter script has failed.

This hasn't impacted uptime of www.m.o but has had a knock on effect with things like sitemap generation, which uses the exported DB.

Over time, these teething troubles with the DB export will disappear, but regardless, we may still have a regression in the future. So, we should add a Dead Man's Snitch and/or Senty logging to the db export script so that we get told sooner of failures.