Error handler

The user can use exception_handler/3 to define his/her own error handler. For example, the following clause makes the system treat calls to an undefined predicate as fail:

		  exception_handler(undefined_predicate,_,fail).

When an error occurs, the system first executes the call:

		  exception_handler(Type,Goal,Handler)

If this call succeeds, the handler Handler defined by the user is executed; otherwise, the default handler is executed.