I have entered some error checking formulas into a worksheet to ensure the
user is aware that someone was missing or entered incorrectly. However, the
user must look at the error columns to see the messages (they can filter on
the columns for the messages).

If the user forgets to look at the messages, is there a way to display a
popup warning message for errors? I would like to stop them from saving
the sheet with errors.

I tried using data validation, but it appears to require a user to manually
enter values. I'm already using the max conditional formatting for other
purposes.

Any tips?