use warning for domain DNS fails

cron.py

@@ -394,7 +394,7 @@ def check_custom_domain():
             # send alert if fail for 5 consecutive days
             if custom_domain.nb_failed_checks > 5:
                 domain_dns_url = f"{URL}/dashboard/domains/{custom_domain.id}/dns"
-                LOG.exception("Alert %s about %s", user, custom_domain)
+                LOG.warning("Alert %s about %s", user, custom_domain)
                 send_email_with_rate_control(
                     user,
                     AlERT_WRONG_MX_RECORD_CUSTOM_DOMAIN,