Checks a local directory and reports any items that:
haven't changed in a while
haven't previously been reported
This class is suitable for calling scanNow() multiple times or just once.
If the stupid method in the AbstractFileSet class returned a
FileScanner interface instead of a DirectoryScanner impl like it should,
then we'd just be implementing FileScanner here.
Extends BuildException with a marker class to indicate that this
build failure was not due to an error in the traditional sense, but rather
due to a normal condition in the course of executing a task.