xds-k8s: Fix assertRpcsEventuallyGoToGivenServers not raising (#30224 An error occurred )

All tests that use `assertRpcsEventuallyGoToGivenServers` method were
reporting successes when the assertion failed:

- FailoverTest
- ChangeBackendServiceTest
- RemoveNegTest

tools/run_tests/xds_k8s_test_driver/framework/xds_k8s_testcase.py

@@ -259,10 +259,11 @@ class XdsKubernetesBaseTestCase(absltest.TestCase):
         try:
             retryer(self._assertRpcsEventuallyGoToGivenServers, test_client,
                     servers, num_rpcs)
-        except retryers.RetryError:
+        except retryers.RetryError as retry_error:
             logger.exception(
                 'Rpcs did not go to expected servers before timeout %s',
                 _TD_CONFIG_MAX_WAIT_SEC)
+            raise retry_error
 
     def _assertRpcsEventuallyGoToGivenServers(self, test_client: XdsTestClient,
                                               servers: List[XdsTestServer],