For any reason, if you don’t want to show the Error Number, Severity Level and the State Number along with the Error Message, use the Severity Level 0 (Zero) or 10. The Severity Number 10 is converted to Zero internally.
Demonstration:
A severity number except 0 or 10 displays the Error Number, Severity and State information.
A severity number 0 or 10 suppresses the Error Number, Severity and State information.
Further Reading:
Database Engine Error Severities
http://msdn.microsoft.com/en-us/library/ms164086(v=sql.105).aspx