Callback method, called if an authorisation error prevents that the
file is checked for existence and last modification date.
Callback method, called if the file is checked but cannot be found.
This method is called the first time the file is determined not to
exist, but also each consecutive time the file is still determined
not to be found.
Callback method, called if the file is found for the first time since
the FileWatcher was started or after it has been deleted.
Each consecutive time the file still exists (and is readable), either
fileModified() or fileNotModified() is called.