onErrorUpdate
The instruction(s) to be executed are placed within quotes. This event handler will execute the instruction(s) when an error has occurred in the transition from a data bound object to a data source.

The following example will display text of your choosing in a message box when an error has occurred in the transition from a data bound object to a data source:
onErrorUpdate="msgWindow.document.write('-- Insert text message here --')"
This Event Handler is used with the following:

APPLET
AREA
BODY
CAPTION
FIELDSET
OBJECT
SELECT
TEXTAREA