[web/views] fix editcontroller regression
unittest_views_basecontrollers.py's
EditControllerTC.test_edit_mandatory_inlined3_object regressed with
changeset dcbb64d3a1d9 because it relied on POSTing nonexisting eids.
This seems like a silly idea, but let's keep the behaviour unchanged.
raise NotImplementedError("Cannot migrate such an old version. Use intermediate Cubiweb version (try 3.16.x)")