Enable an app to display a custom "interrupted" dialog

Rather than just allowing the 'The download of {filename} was interrupted' system message box text to be customized, enable the app to provide a dialog box styled per the app.

Could perhaps be achieved by one of: (a) Add an 'onError' optional callback. (No "interrupted" dialog is displayed is this callback is specified.) (b) Allow a blank string to be specified as the value of 'errorMessage'. (No "interrupted" dialog is displayed if the string is ""; the "interrupted" result can be found by testing the 'state' returned by the 'done' event).