How to return an ERROR using functions

When a function doesn't have the data it needs, or is set up incorrectly, it returns an error icon (the red triangle with an exclamation mark). How do I intentionally return this error using functions?

I used to use IF statements to return an error by leaving the IFFALSE field blank. This worked for my needs until I built one that returned 0. I read that apparently if you leave IFFALSE or IFTRUE blank, it will correspondingly return 0 if those conditions are met.