fix(dns): retry configurable resolvers

Treat resolver transport failures as inconclusive checks so they do not open outage alerts.

checks/cdns/dns.go

@@ -73,7 +73,9 @@ func Perform(c *models.Check) *Result {
 	go localQuery(nsChan, zone, dns.TypeNS)
 	nsResult := <-nsChan
 	if nsResult.r == nil {
-		result.State = stateERR
+		// All recursive resolvers were unreachable, so this worker could not
+		// establish whether the domain is healthy. Do not open an outage event.
+		result.State = "FAIL"
 		result.Error = fmt.Errorf("cannot retrieve the list of name servers for %s: %s", zone, nsResult.err)
 		return result
 	}