Documented structure error messages

I have reviewed and unified all error messages and placed them in the
README file. At last they can be viewed in one place.

Also, error messags for toplevel clauses were made more elaborate.
Updated and unified expected error messages

- Updated error messages to match ones in README
- Updated toplevel test to keep up with README

Also, I have pushed more work on toplevel partitioning: they now
check the structure of (extends ...) etc. clauses and extract
the names. Result is either #f or (name)--to catch things like
(extends #f) which must not conflict with our 'not specified #f'.