Update error messages in sanity tests

Update error messages throughout sanity tests
to use the same (or similar) format of error message.

Change-Id: I4db0517a5b264151ddaa6ea2c2a04d37e139944e
Related-PROD: #PROD-32415
diff --git a/test_set/cvp-sanity/tests/test_single_vip.py b/test_set/cvp-sanity/tests/test_single_vip.py
index cee92ec..f0e7d68 100644
--- a/test_set/cvp-sanity/tests/test_single_vip.py
+++ b/test_set/cvp-sanity/tests/test_single_vip.py
@@ -32,5 +32,7 @@
                 no_vip[group] = 'No vip found'
             else:
                 no_vip[group] = nodes_list
-    assert len(no_vip) < 1, "Some groups of nodes have problem with vip " \
-           "\n{}".format(json.dumps(no_vip, indent=4))
+    assert len(no_vip) < 1, (
+        "The following group(s) of nodes have problem with vip:\n{}".format(
+            json.dumps(no_vip, indent=4))
+    )