Database Engine Severity Levels
You should return 16. Is the default, most used error level:
Indicates general errors that can be
corrected by the user.
Don’t return 17-18, those indicate more severe errors, like resource problems:
Indicate software errors that cannot
be corrected by the user. Inform your
system administrator of the problem.
Also don’t return 11-15 because those have a special meaning attached to each level (14 – security access, 15 – syntax error, 13 – deadlock etc).
Level 16 does not terminate execution.
When your intention is to log a warning but continue execution, use a severity level below 10 instead.