The outputErrors option outputs a native Google Sheets #ERROR! message when an error occurs.
By default the function returns custom messages when an error occurs (such as #SELECTOR_NOT_VALID). Those messages are not recognized by Google Sheets as proper errors. Therefore you cannot use the ISERROR or IFERROR functions.
=IMPORTFROMWEB(data_sources, selectors, "outputErrors")
|3||=IMPORTFROMWEB(data_sources, selectors, A1:B1)|
#ERROR! messages can be handled with the ISERROR or IFERROR functions.
The downside of it is that the #ERROR! message displayed by Google Sheets doesn’t tell much about the type of error. Pass the mouse over the cell to see more information about the error.