I need to capture look up errors (-1 index) when I'm missing good master data in my app. It's creating other records, but I need to know when someone runs it and it throws an error, thereby missing data. 

I tried to ADD a RECORD to a simple Log File form I created, but it's not taking it, I assume because the docs say you can't add a record in the middle of an On Success workflow? The main workflow cannot be changed. It's complicated. It's has to be On Success.

How can I capture those errors so that they can be corrected? Cannot use debugging statements, as it's for when the app is in production with other users.

Thanks.