Review error messages

Review every command defined in repl and clean up their error messages.

For inspiration, review the original Fortran subroutines. Add the name(s) of the relevant subroutines and the files they came from to the function description would also be a good idea.

A list of commands to tick off.

Top level:

  ADD        BACK       BULLETIN   CHANGE     COPY       CREATE
  Ctrl-C     CURRENT    DELETE     DIRECTORY  EXIT       FIRST
  Folders    FORWARD    HELP       INDEX      Keypad     LAST
  MAIL       MARK       MODIFY     MOVE       NEXT       PRINT
  QUIT       READ       REMOVE     REPLY      RESPOND    SEARCH
  SEEN       SELECT     SET        SHOW       SSH        UNMARK
  UNSEEN     USER

SET:

  ACCESS           ALWAYS           BRIEF            DEFAULT_EXPIRE
  EXPIRE_LIMIT     FOLDER           NOACCESS         NOALWAYS
  NOBRIEF          NOPROMPT_EXPIRE  NOREADNEW        NOSHOWNEW
  NOSYSTEM         PROMPT_EXPIRE    READNEW          SHOWNEW
  SYSTEM

SHOW:

  FLAGS       FOLDER     -NEW         PRIVILEGES  USER        VERSION

SSH:

  ADD        DELETE     FETCH      LIST

USER:

  ADD        ADMIN      DELETE     DISABLE    ENABLE     LIST
  MOD        NAME       NOADMIN    NOMOD