Adding User-Defined Error Messages
User-defined error messages can be added to the sysmessages table using the system stored procedure sp_addmessage. At a minimum, you can specify the message number, the severity level, and the message text using sp_addmessage.
Similar to specifying user-defined error messages with RAISERROR, use error message numbers greater than 50000 and severity levels from 0 through 18. Only system administrators can issue RAISERROR with a severity level from 19 through 25.