add IF EXISTS to migrations
shouldn't be necessary unless you manually mess with the DB schema, but
doesn't hurt to have
-DROP TRIGGER workers_upsert_or_delete_trigger ON workers;
-DROP TRIGGER containers_insert_or_delete_trigger ON containers;
+DROP TRIGGER IF EXISTS workers_upsert_or_delete_trigger ON workers;
+DROP TRIGGER IF EXISTS containers_insert_or_delete_trigger ON containers;
-DROP FUNCTION notify_trigger;
+DROP FUNCTION IF EXISTS notify_trigger;
