raise an AssertionError if do_barrier fails

This removes a lot of boilerplate code.
diff --git a/tests/openflow_protocol_messages.py b/tests/openflow_protocol_messages.py
index 4f63b07..b182015 100644
--- a/tests/openflow_protocol_messages.py
+++ b/tests/openflow_protocol_messages.py
@@ -265,7 +265,7 @@
 
         #Clear Switch state
         delete_all_flows(self.controller)
-        self.assertEqual(do_barrier(self.controller), 0, "Barrier failed")
+        do_barrier(self.controller)
 
         logging.info("Sending a Simple tcp packet a dataplane port")
         logging.info("Expecting a packet_in event on the control plane")