What severity are you using in your RAISERROR?

It sounds like you don't have your BEGIN/END statements placed properly.  You should never be able to process both sides of an IF...THEN...ELSE statement.

Also, I would use IF NOT EXISTS ( <subquery> ) rather than IF ( <subquery> ) IS NULL, for several reasons.
1) It's more widely used.
2) It's more accurate.  It can distinguish between failing to return a record and returning a record whose column contains a NULL value.
3) It doesn't fail if the subquery returns multiple records
4) It doesn't fail if the subquery returns multiple fields.

Drew