excel spreadsheets / blank cells

I'm trying to find blank cells and issue a warning message.

I have tried the following.

(1) Conditional Formating
Worked, but this only highlights the cell in red no warning.

(2) Cell formula IF(ISBLANK(CELL), "error",)
gives me a circular reference error.

Does anyone know a way to do this?
It seems like an easy task, but it has escaped me.

Are you doing this using VBA or I guess just straight excel formula logic, huh?

all excel formula logic.
i have tried the following combinations.

Code:

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=IF(ISBLANK(A5),"error",A5) //circular reference
=IF(A5="","error",A5)// circular reference

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is it possible to only run those checks when the file is saved??