Fix search string for error in test_manually_trigger_update_secondary_zone_negative

Socket library returns  when
can't resolve host.

Related-Prod: PRODX-25653
Change-Id: Iefd749e614688300d5d1a87e845f3067e89742f7
diff --git a/designate_tempest_plugin/tests/api/v2/test_zone_tasks.py b/designate_tempest_plugin/tests/api/v2/test_zone_tasks.py
index 70a2e77..38a17e0 100644
--- a/designate_tempest_plugin/tests/api/v2/test_zone_tasks.py
+++ b/designate_tempest_plugin/tests/api/v2/test_zone_tasks.py
@@ -224,5 +224,5 @@
         # Manually trigger_update zone
         LOG.info('Manually Trigger an Update of a Secondary Zone when the '
                  'nameservers not pingable. Expected: error status code 500')
-        with self.assertRaisesDns(lib_exc.ServerFault, 'unknown', 500):
+        with self.assertRaisesDns(lib_exc.ServerFault, 'socket.gaierror', 500):
             self.client.trigger_manual_update(sec_zone['id'])