Misses proper documentation/handling of what to do when a stored value does not match requirements any more

[do3cc]

We get a traceback that ends like this:

  File "home/starzel/.buildout/eggs/z3c.form-3.0.5-py2.7.egg/z3c/form/field.py", line 277, in update
    widget.update()
  File "home/starzel/.buildout/eggs/z3c.formwidget.query-0.10-py2.7.egg/z3c/formwidget/query/widget.py", line 152, in update
    terms.append(source.getTerm(value))
  File "home/starzel/.buildout/eggs/plone.formwidget.contenttree-1.0.7-py2.7.egg/plone/formwidget/contenttree/source.py", line 130, in getTerm
    % str(value))

We are using an UUID Binder. The object the UUID is refering to does still exist, but the UUIDBinder has some constraints that this object does not contain anymore. In hindsight, one should not add such a filter. In hindsight