Better errors in large-anon parsing code

[phadej]

Currently the parsing code in large-anon (i.e. code which elaborates constraints/givens etc.) uses Maybe as it's main monad.

This means that we either get Just result (good) or machine says No (Nothing). This is fine for the end user but somewhat annoying when developing plugin itself.

Using even Either SDoc is already an improvement, but we can do better to support few Alternative uses.