If there is a reference to a non existent named cell in a formula the following exception is thrown:
"Specified named range 'NAMEVAR' does not exist in the current workbook."

This is also the case if the name is wrapped inside a "ISERROR" or "IFERROR".

There we would expect the same behavior as in Excel, also even if it is just inserted I'd expect a Name error in the Cell and not a runtime exception, essentially the same behavior that is present if a formula is set to divide by 0.

Also there if there is a reference to an non existing sheet or external workbook then one can use:
>FormulaEvaluator#setIgnoreMissingWorkbooks()
to get Excels behavior, but named cells will still fail.

Code to reproduce:
>Workbook workbook = new XSSFWorkbook();
>Cell cell = workbook.createSheet().createRow(0).createCell(0);
>cell.setCellFormula("ISERROR(NAMEVAR)");