Clarify code
Rewrite README
Clarify error messages

Now I get a different error message:

    $ ministg sum.stg
    Error reading from file:Prelude.stg
    Prelude.stg: openFile: does not exist (No such file or directory)

It took me a while to realize that "Prelude.stg" was missing. I first
thought it just meant that openFile was from Haskell's prelude. Very
confusing.