Fix bad error for non-existent methods on a class

When defining an instance for Monad with a silly method:

    instance Monad X where
      fakeMethod = 100

Would say:

> Type class Prelude.Monad is undefined. Type class names must be
  qualified.

It now says:

> Type class Prelude.Monad does not have method fakeMethod
Merge pull request #159 from puffnfresh/superclasses

Fix bad error for non-existent methods on a class