bsn_gentable: expect no reply when deleting a nonexistent message

Behavior changed by https://github.com/floodlight/loxigen/pull/193
diff --git a/tests-1.3/bsn_gentable.py b/tests-1.3/bsn_gentable.py
index e9f3145..915729c 100644
--- a/tests-1.3/bsn_gentable.py
+++ b/tests-1.3/bsn_gentable.py
@@ -702,8 +702,7 @@
         do_barrier(self.controller)
 
         error, _ = self.controller.poll(ofp.OFPT_ERROR, 0)
-        self.assertIsInstance(error, ofp.message.bad_request_error_msg)
-        self.assertEquals(error.code, ofp.OFPBRC_EPERM)
+        self.assertEquals(error, None)
 
 class DeleteFailureError(BaseGenTableTest):
     """