nsIDOMDOMException

In general, DOM methods return specific error values in ordinary
processing situations, such as out-of-bound errors.
However, DOM operations can raise exceptions in “exceptional”
circumstances, i.e., when an operation is impossible to perform
(either for logical reasons, because data is lost, or because the
implementation has become unstable)

For more information on this interface please see
http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#domexception