missing variable error in abstract closures should suggest adding the variable to the captures list, if applicable

[bakkot]

e.g.

1. Let _foo_ be 0.
1. Let _AC_ be a new Abstract Closure with no parameters that captures nothing and performs the following steps when called:
  1. Return _foo_.

This is an error because the AC uses but doesn't capture _foo_, but the error is just "could not find a preceding declaration for _foo_". If the thing being used exists outside the AC the error should suggest adding it to the captures list.