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_oss.py b/test_set/cvp-sanity/tests/test_oss.py
index 469323e..f28917c 100644
--- a/test_set/cvp-sanity/tests/test_oss.py
+++ b/test_set/cvp-sanity/tests/test_oss.py
@@ -39,6 +39,7 @@
                 if row['svname'] != 'FRONTEND' and row['status'] != 'UP':
                         wrong_data.append(info)
 
-    assert len(wrong_data) == 0, \
-        '''Some haproxy services are in wrong state
-              {}'''.format(json.dumps(wrong_data, indent=4))
+    assert len(wrong_data) == 0, (
+        "Some haproxy services are in wrong state:\n{}".format(
+            json.dumps(wrong_data, indent=4))
+    )