## Improve error message about closed variables

After allowing local bindings in static definitions the following program

```
{-# LANGUAGE StaticPointers #-}
module M where
f x = static g
where
g = h
h = x
```

yields the error

```
../tmp/M.hs:4:7: error:
• ‘g’ is used in a static form but it is not closed.
• In the expression: static g
In an equation for ‘f’:
f x
= static g
where
g = h
h = x
```

where it would be more helpful to get a message like

```
../tmp/M.hs:4:7: error:
• ‘g’ is used in a static form but it is not closed because it
uses ‘h’ which uses ‘x’ which is not let-bound.
• In the expression: static g
In an equation for ‘f’:
f x
= static g
where
g = h
h = x
```