Further custom type error questions

[Ben Gamari]

While preparing some additional documentation for Iavor's custom type
errors work (which has been merged; thanks Iavor!) I noticed that
Dominique Devriese has raised some additional questions on the proposal
[1].

In particular, Dominique suggests that perhaps TypeError should simply
be of kind `ErrorMessage -> Constraint`. My understanding of the
proposal is that the intention is that `TypeError`s should be usable on
the RHS of type functions, like `error` on the term level. However, is
this strictly necessary? Is there any place where you couldn't just as
easily express the `TypeError` as a constraint?

If not, it seems like this may be substantially simpler approach and
totally side-steps the detection of `TypeError` in inappropriate places
on the RHS.

Regardless, it seems like this (and the other questions) is worth
addressing in the proposal.

Cheers,

- Ben


[1] https://ghc.haskell.org/trac/ghc/wiki/Proposal/CustomTypeErrors#SomedesignquestionsDominiqueDevriese:
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