Haddock reader: improved error reporting.
Haddock reader: add the "haskell" attribute to code blocks so they are
highlighted. We reasonably assume that code blocks in Haddock markup
wont be used for anything but Haskell code.
Haddock reader: make clearer which production the comment refers to.
Merge pull request #808 from davidlazar/master

Improvements to Haddock reader