Confusing error message for unused variables in RHS of data family instance
```haskell
{-# LANGUAGE TypeFamilies #-}
module T23778 where
import Data.Kind
data family D :: Type
data instance forall (d :: Type). D = MkD
```
On GHC 9.6 I get
```
* Type variable `d' is mentioned in the RHS,
but not bound on the LHS of the family instance
```
The message was reworded slightly in 9.8 with the error constructor migration, but it's similar:
```
* Out of scope type variable `d' in the RHS of a family instance.
All such variables must be bound on the LHS.
```
Instead I would expect to get an error that matches the one we get for type family instances
```haskell
type family F
type instance forall (a :: Type). F = ()
```
```
* Type variable `a' is bound by a forall,
but not used in the family instance.
```
issue