Tweak error messages for narrowly-kinded assoc default decls

This program, from #13971, currently has a rather confusing error
message:

```hs
class C a where
  type T a :: k
  type T a = Int
```
```
    • Kind mis-match on LHS of default declaration for ‘T’
    • In the default type instance declaration for ‘T’
      In the class declaration for ‘C’
```

It's not at all obvious why GHC is complaining about the LHS until
you realize that the default, when printed with
`-fprint-explicit-kinds`, is actually `type T @{k} @* a = Int`.
That is to say, the kind of `a` is being instantiated to `Type`,
whereas it ought to be a kind variable. The primary thrust of this
patch is to weak the error message to make this connection
more obvious:

```
    • Illegal argument ‘*’ in:
        ‘type T @{k} @* a = Int’
        The arguments to ‘T’ must all be type variables
    • In the default type instance declaration for ‘T’
      In the class declaration for ‘C’
```

Along the way, I performed some code cleanup suggested by @rae in
https://gitlab.haskell.org/ghc/ghc/issues/13971#note_191287. Before,
we were creating a substitution from the default declaration's type
variables to the type family tycon's type variables by way of
`tcMatchTys`. But this is overkill, since we already know (from the
aforementioned validity checking) that all the arguments in a default
declaration must be type variables anyway. Therefore, creating the
substitution is as simple as using `zipTvSubst`. I took the
opportunity to perform this refactoring while I was in town.

Fixes #13971.

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