structured error messages

[John Ericson]

> Ultimately, if reading error messages did require linking with GHC, it 
> wouldn't be the end of the world.  I would just write my own 
> translation layer to JSON.  So consider this a weak preference.

I think this is the key. Ultimately, all other uses can be done on top 
of just linking GHC the library. Just as we have ghc-lib to paper over 
the differences between GHC versions and save individual projects from 
developing their own comparability layers, so we could also have some 
ghc-json library and tweaked frontend executable or something.

The GHC repo has stringent CI and is somewhat a bottleneck already, so 
the more we can do out-of-band, the better, I say.

John