make Haskell tests return a proper exit code on failure

This fixes all of the Haskell tests so that the exit code makes sense. Should help with https://github.com/kytrinyx/exercism/pull/79 and similar efforts. I'm also looking into what it will take to get these tests run by Travis. I have a script working locally, but Travis doesn't really do multi-language and the Haskell it has is too old anyway so I have to build some compatible binaries to make that work.