Use a better string

jedi/evaluate/syntax_tree.py

@@ -102,7 +102,7 @@ def eval_node(context, element):
         # Must be an ellipsis, other operators are not evaluated.
         # In Python 2 ellipsis is coded as three single dot tokens, not
         # as one token 3 dot token.
-        assert element.value in ('.', '...'), element.value
+        assert element.value in ('.', '...'), "The unhandled operator %s" % repr(element.value)
         return ContextSet(compiled.builtin_from_name(evaluator, u'Ellipsis'))
     elif typ == 'dotted_name':
         context_set = eval_atom(context, element.children[0])