Poor error message for misnamed packed in cabal file

[erikd-ambiata]

When for instance I add resource (when I meant to add resource) to a cabal file, mafia gives a really horrible error message.

I shouldn't be too hard to check all the package names against the global cabal package name list and the locally defined submodules and give a much better error message.

[ggranberry]

I was hoping to start contributing to Mafia. Would this be a good piece of low-hanging fruit for me to start out with?

[erikd]

This one probably is not an easy one, but #178 probably is.