Stop quickly _getOb uses of asComposedDocument() result

In predicate searches using asComposedDocument() result as context, categories where being looked up as subobjects, triggering the _folder_handler assertion.
Make sure it fails the lookup fast.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@45049 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/mixin/composition.py

@@ -144,7 +144,10 @@ class asComposedDocument(object):
 
   @property
   def _folder_handler(self):
-    assert False
+    assert False, "Attempt to use .asComposedDocument() result as folder. This should never happen!"
+
+  def _getOb(self, key, *args, **kw):
+    raise KeyError(key)
 
   def __getattr__(self, name):
     raise AttributeError(name)