Catch it from where? In T-SQL or an app language like C#?

If your supplying a severity level of 10 then it will not trip a T-SQL CATCH block. Your RAISERROR with severity 10 is equal to a PRINT statement.

If connecting to SQL Server from .NET you can capture everything coming from the INFO message stream which is where PRINT and warning (i.e. RAISERROR with severity 10 or less) messages are written. If you're using .NET look into the SqlInfoMessageEventHandler class.