John Mitchell-245523 (1/20/2015)
The allow updates option doesn't have any effect any more (from SQL Server 2005). You could use a trigger instead of a constraint in order to return the error message you want, but this is inefficient and I would only use it as a last resort.John
You are right. It is impossible now.
As most of SQL experts would say:
Never ever ever, change system tables.
...apart of - you can add new user defined messages to sys.messages using sp_addmessage...