Fix error reporting for inventory queries

Inventory query errors were raising exceptions due to a missed
change of a value.contents() method to a class property
diff --git a/reclass/core.py b/reclass/core.py
index 75eea54..6dac5c3 100644
--- a/reclass/core.py
+++ b/reclass/core.py
@@ -189,7 +189,7 @@
                             node.interpolate_single_export(q)
                         except InterpolationError as e:
                             e.nodename = nodename
-                            raise InvQueryError(q.contents(), e, context=p, uri=q.uri)
+                            raise InvQueryError(q.contents, e, context=p, uri=q.uri)
                 inventory[nodename] = node.exports.as_dict()
         return inventory