action_list: throw exception if value is not an action

This is a programmer error, not a switch issue.
diff --git a/tests/flow_stats.py b/tests/flow_stats.py
index 040fec8..da12071 100644
--- a/tests/flow_stats.py
+++ b/tests/flow_stats.py
@@ -144,7 +144,7 @@
         flow_mod_msg.hard_timeout = 65000
         flow_mod_msg.priority = 100
         act.port = egress_port
-        self.assertTrue(flow_mod_msg.actions.add(act), "Could not add action")
+        flow_mod_msg.actions.add(act)
        
         # send flow
         logging.info("Inserting flow")
@@ -197,7 +197,7 @@
         flow_mod_msg.hard_timeout = 0
         act = action.action_output()
         act.port = egress_port
-        self.assertTrue(flow_mod_msg.actions.add(act), "Could not add action")
+        flow_mod_msg.actions.add(act)
 
         logging.info("Ingress " + str(ingress_port) + 
                        " to egress " + str(egress_port))
@@ -325,7 +325,7 @@
         flow_mod_msg.hard_timeout = 0
         act = action.action_output()
         act.port = egress_port
-        self.assertTrue(flow_mod_msg.actions.add(act), "Could not add action")
+        flow_mod_msg.actions.add(act)
 
         logging.info("Ingress " + str(ingress_port) + 
                        " to egress " + str(egress_port))
@@ -504,7 +504,7 @@
         flow_mod_msg.priority = 100
         flow_mod_msg.flags = ofp.OFPFF_SEND_FLOW_REM
         act.port = egress_port
-        self.assertTrue(flow_mod_msg.actions.add(act), "Could not add action")
+        flow_mod_msg.actions.add(act)
 
         # send flow
         logging.info("Inserting flow")