Improve the error message from unifyFunTys.  Previously we got a really
horrible message from this:
	  t = ((\Just x -> x) :: Maybe a -> a) (Just 1)
    
Try.hs:1:6:
    Couldn't match the rigid variable `a' against `t -> t1'
      Expected type: a
      Inferred type: t -> t1

Now it's much better:

Try.hs:14:6:
    The lambda expression `\ Just x -> ...' has two arguments,
    but its type `Maybe a -> a' has only one
    In the expression: (\ Just x -> x) :: Maybe a -> a


tcfail140 tests some cases