public class CancelException
An exception that is not reported to the user. This exception can be used
to cancel an ongoing operation. It works together with MessageHandler,
where it might be thrown when the user clicks "cancel" in a dialog box.
Throwing an exception allows the execution environment to unwind the stack,
cancelling operations that may only lead to an endless stream of warnings
or errors. This class is really just a marker, and is intended to not be
reported to the user.