Future work: making parse errors more specific

> Perhaps future work could be making parse errors more specific; one error number for all of them seems like it would make `errors.haskell.org` less useful.

_Originally posted by @geekosaur in https://github.com/haskell/cabal/pull/10525#discussion_r1849065788_

Start with the discussion in pull request #10525, where this future work was originally proposed. Determine which parse errors are currently grouped under one error number and define the desired specificity before identifying the affected Cabal code and tests. Done means parse errors have useful distinct classifications for errors.haskell.org.

Written by the indexing model from the issue text.