action_list: throw exception if value is not an action

This is a programmer error, not a switch issue.
diff --git a/tests/flow_expire.py b/tests/flow_expire.py
index ad35f23..93df5f8 100644
--- a/tests/flow_expire.py
+++ b/tests/flow_expire.py
@@ -65,7 +65,7 @@
         request.idle_timeout = 1
         request.flags |= ofp.OFPFF_SEND_FLOW_REM
         act.port = egress_port
-        self.assertTrue(request.actions.add(act), "Could not add action")
+        request.actions.add(act)
         
         logging.info("Inserting flow")
         rv = self.controller.message_send(request)