Part of the contract-error fix is an implementation change, and part is a documentation change. Closes PR 12904
due to recent conversion from `mzscheme' to `racket/base'
Although the ".plt" format is going to be replaced, the format is currently viable for distributing collections, and I have wanted a raw `unpack' command for a while. It was useful today to fix problems with `raco pack' and collection links.
