identifier resolve ast.undefined does not throw exception.

[Berserker66]

At the end of resolve_identifier of LiteralResolver it currently just returns ast.Undefined().
As far as I can tell, there is no method to check for any undefined values. In my particular project I replaced that line with "raise AttributeError("{} could not be resolved.".format(identifier))" as any unresolved identifier would be an error.

Either throwing an error or giving a method to check for unresolved identifiers would be useful.