raise an AssertionError if do_barrier fails

This removes a lot of boilerplate code.
diff --git a/tests/bsn_mirror.py b/tests/bsn_mirror.py
index ebf0f75..e2d895e 100644
--- a/tests/bsn_mirror.py
+++ b/tests/bsn_mirror.py
@@ -152,7 +152,7 @@
         flow_mod.actions.add(act3)
         delete_all_flows(self.controller)
         self.controller.message_send(flow_mod)
-        self.assertEqual(do_barrier(self.controller), 0, "Barrier failed")
+        do_barrier(self.controller)
         
         logging.info("Sending packet to port %s" % ports[0])
         self.dataplane.send(ports[0], str(pkt))