11109 chgrp fails on links

Both http://trac.openmicroscopy.org.uk/ome/ticket/11109 and http://trac.openmicroscopy.org.uk/ome/ticket/11118 seem to be caused by the loss of the `FORCE` setting when validation graph steps.

This commit further refactors the `validate()` method (as previously suggested by @mtbc.

Please see those tickets for testing instructions.

/cc @gusferguson, @pwalczysko, @jburel

Bugs related:
 * http://trac.openmicroscopy.org.uk/ome/ticket/11078
 * http://trac.openmicroscopy.org.uk/ome/ticket/11109
 * http://trac.openmicroscopy.org.uk/ome/ticket/11118