OnError is used internally by the IDE to report errors. It can also be used to write an event handler to selectively choose to process or ignore errors. 

The last parameter passed to the OnError event handler, Handled, is passed by reference. By default, the Error method passes false in Handled, but a handler that corrects the error or chooses to ignore the error can set Handled to true, which prevents further processing of the error. If the event handler returns with Handler still set to false, the reader object raises an EReadError exception. 

EReadError