Unbound variable in a pattern synonym

This

```haskell
{-# LANGUAGE PatternSynonyms #-}
pattern P x = ()
```

gives

```
M.hs:2:11: error: Not in scope: ‘x’
|
2 | pattern P x = ()
| ^
```

I think this is too terse and the error should give more explanation. For a newcomer, it's very natural to expect it to work, because `f x = ()` works.

For comparison,

```haskell
pattern Q = x
```

gives

```
M.hs:2:13: error:
Invalid right-hand side of bidirectional pattern synonym ‘Q’:
‘x’ is not bound by the LHS of the pattern synonym
RHS pattern: x
|
2 | pattern Q = x
| ^
```

This is better, though it could also suggest adding 'x' to the LHS, or changing it to a wildcard, or making the pattern synonym unidirectional.