No diagnostic for misplaced LANGUAGE pragma

If I say

module Main where

{-# LANGUAGE ScopedTypeVariables #-}

I get no diagnostic. Instead, if I, say, try to use a scoped type variable, I'll just get errors -- all the more confusing because I thought I enabled scoped type variables.

This should be an error, not a warning, because this error is likely to cause others.

Interestingly, if I misspell LANGUAGE, then I get a warning.

This is spun off from https://github.com/haskell/error-messages/issues/21.