Using items not defined in the instance

[bderenzi]

If you don't define an item in the instance, but still use it later on, the parser doesn't care. Seems like we should force that things are in the instance otherwise there's no point to having an instance. Caused a minor headache for me because the instance had "client_missing" but then I used "missing_client" and couldn't figure out why my relevants weren't working.