[PDI-10270] Fix: do not set result.nrErrors in case of 'normal' fail of ...

[PDI-10270] Fix: do not set result.nrErrors in case of 'normal' fail of a particular Job Entry

* for the sake of backward compatibility special Kettle variable is introduced:
	KETTLE_COMPATIBILITY_SET_ERROR_ON_SPECIFIC_JOB_ENTRIES
	if this variable is set to "Y" old behavior takes place.

* result.nrErrors is set only in case of exceptional flow:
	e.g. bad arguments (null), non existing directory to be checked by the Job Entry etc.

* unit tests implemented for both FilesExist and FolderIsEmpty job entries.