Resurrect the %errorhandlertype directive for back-compat (#320)

[sgraf812]

Fixes #320.

[Bodigrim]

FWIW happy-2.1 was a major bump already, which is enough for a breaking change (even if not reflected in the changelog).

[sgraf812]

Thanks. Indeed I was wondering whether happy's versioning scheme is subject to the PVP (it appears that every package is). I bumped to 2.1.1 instead.

I brought back the old %errorhandlertype feature (and will now go wash my hands).

[sgraf812]

@yav I verified that the package candidate 2.1.1 makes language-rust buildable again. I'll merge soon. Try with the following cabal.project:

packages:
  .
  http://hackage.haskell.org/package/happy-lib-2.1.1/candidate/happy-lib-2.1.1.tar.gz
  http://hackage.haskell.org/package/happy-2.1.1/candidate/happy-2.1.1.tar.gz