Don't emit utilities containing invalid theme fn keys

[thecrypticace]

Fixes #9318

This fix isn't ideal — need to iterate on it some before merging. We shouldn't even generate the utility in the first place. The problem here is the utility and the evaluation of the theme() function are at two separate stages. This causes the utility to be stored in the map when it is otherwise technically invalid. As a perf optimization we don't clear out the utility list when "watching" — it's effectively append-only causing this to throw as soon as you accidentally introduce a utility w/ a theme key that doesn't exist.

[thecrypticace]

Hey @RobinMalfait how bad of an idea is this — or does this seem fine-ish?

[RobinMalfait]

Hmm, I see what you mean. Instead of silently ignoring I would expect an error/warning being thrown instead so that you have at least something to debug (or open an issue about).

I think in the past we've handled the resolving of the theme function with special cases for some plugins. Ideally we figure out a way to resolve the theme value value per-plugin but that's probably out of scope for this one.

I think what we can do here is:

1. Throw some form of error in the general case when theme function doesn't resolve to a proper (string?) value.
2. Handle the boxShadow properly if that's not the case already (doubt it).

I think we've used throw node.error('Message') before which should be recoverable (the process should not crash but an error should be shown in the terminal)