Tests for an error value other than #N/A.
Returns TRUE if value refers to or evaluates to an error value other than the Not Available error #N/A, and FALSE otherwise.
Use the ISERROR function to test for any errors including #N/A.
returns TRUE, because taking the squareroot of -1 is an error.
where C5 contains 123, returns FALSE, because 123 is not an error.
returns FALSE, because the error value #N/A is ignored by this function.