Change expected values for the Natural constraint

integration/test_pn_interface.py

@@ -101,8 +101,8 @@ help_root =         ["Currently no model is loaded, so your operations are limit
                      "  delete -- Delete a previously made model",
                      "  list   -- Show a list of all stored models",
                      "  help   -- Show a list of possible commands"]
-verify_fail_weight= ["Negative weight in arc p2t"]
-verify_fail_tokens= ["Negative number of tokens in Place p1"]
+verify_fail_weight= ["Natural number not larger than or equal to zero"]
+verify_fail_tokens= ["Natural number not larger than or equal to zero"]
 verify_fail_structure = ["Source of model edge not typed by source of type: p2t"]
 
 init = greeting + prompt