I made a small utility to do this programmatically. Assuming your formula returns correct results and has correct syntax, this utility adds an ISERROR OR ISNA to cell error values. It also allows for specifying text or characters to be used in the errors place, and for the ISEROR and ISNA to be programmatically removed from the formula
You can download it here
http://jlxl.net/Excel/downloads.html