Display Haddock diagnostics

[michaelpj]

Is your enhancement request related to a problem? Please describe.

Haddock produces various warnings that are somewhat useful. For example, it will tell you if it cannot figure out where to link something to, which often indicates a typo.

These warnings are rarely noticed, because the only time you see them is when you run cabal haddock (and the format is terrible). Surfacing them in HLS would be pretty helpful.

Describe the solution you'd like

Haddock warnings (and errors?) appear in HLS like normal diagnostics.

Additional context

I have no idea how hard it is to get these. I think we are running haddock (if documentation: true is set), so presumably we should be able to get the warnings too.

In addition, I notice that the warnings printed out by haddock on the CLI don't even have locations (!!!), so we might have to work on Haddock to get the information we need.