fix #<undefined> mention and adjust example to always terminate
mention that all definitions are exported in #lang plai's docs
adjust drracket test for the new error message format
add some string constants to be used by the gui pkg manager
document gui-pkg-manager and add pkg-catalog-update-local/simple-status-dialog
adjust DrRacket to be able to populate the local pkg catalog
database in more convenient ways

- add a "update cache" button to online check syntax error message when
  a missing module exn is raised, but there are no suggestions
- add an "update cache" link to the missing module exn printout
fix struct/dc chaperone/flat/impersonator-ness when #:inv is used