action_list: throw exception if value is not an action

This is a programmer error, not a switch issue.
diff --git a/tests/bsn_mirror.py b/tests/bsn_mirror.py
index c0ecc07..fd211c8 100644
--- a/tests/bsn_mirror.py
+++ b/tests/bsn_mirror.py
@@ -149,9 +149,9 @@
         act3.port = ports[1]
         flow_mod = message.flow_mod()
         flow_mod.match = match
-        self.assertTrue(flow_mod.actions.add(act1), "Could not add mirror action")
-        self.assertTrue(flow_mod.actions.add(act2), "Could not add mirror action")
-        self.assertTrue(flow_mod.actions.add(act3), "Could not add output action")
+        flow_mod.actions.add(act1)
+        flow_mod.actions.add(act2)
+        flow_mod.actions.add(act3)
         self.assertEqual(delete_all_flows(self.controller), 0,
                          "Failed to delete all flows")
         self.assertNotEqual(self.controller.message_send(flow_mod), -1,