Inky fails to run without any error message when undeclared EXTERNAL functions exist.

If you have EXTERNAL functions referenced in your script that don't have a fallback, calling them anywhere in your story prevents inky from showing anything in the right-hand player window, and doesn't show a warning or anything else to hint at the issue.

Struck me as something to fix (perhaps writing blank fallback functions invisibly+automatically in inky when an EXTERNAL is defined without a fallback?), or just adding a warning.

```-> Intro
= Intro

Intro story text!
+ Go to the shop
~ OpenShop()
->Intro

EXTERNAL OpenShop()
// Here's the issue. If you remove this, it won't run in inky, nor will it give any indication of why not.
=== function OpenShop()
Open Shop```