add error handling formulas

I could not find documentation for `ISERR`, but the help text in Excel says it checks for everything except `#N/A`. [`ISERROR`](http://office.microsoft.com/en-001/excel-help/iserror-function-dax-HA102838076.aspx) checks for everything `ISERR` does, plus `#N/A`. Both of them return booleans.

[`IFERROR`](http://office.microsoft.com/en-001/excel-help/iferror-function-HA001231765.aspx) returns the second argument if the first one is an error.