sp_grantlogin Error Hanlde When NT username not valid

If I'm creating (sp) that Adding NT user to the Data base I should do like this:

exec sp_grantlogin 'Servername\username'

But suppose the user name is not inside the NT security names then the following error will be displayed :

MSG 15401 : Windows NT user or group 'Servername\username' not found check the name again.

How I can handle this error when The User name is not found in the NT security users , I mean if I do like this :

exec sp_grantlogin 'Servername\username'
if @@error <> 0

I did it like this but it didn't works , any body can help me how to handle it?
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sp_grantlogin returns 0 for success or 1 for failure. You could try this:

DECLARE @rc int

EXEC @rc = sp_grantlogin 'Servername\username'
IF @rc = 0 -- success
ELSE -- failure

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