bug#29842: 27.0; `dired-change-marks': args should not be optional

[Drew Adams]

Furthermore, the code should perhaps not just call
`ding' if you hit RET.  Instead of this:

 (if (or (eq old ?\r) (eq new ?\r))
     (ding)
  ...)

it should perhaps do something like this:

 (unless (char-displayable-p old)
   (error "Not a displayable character: `%c'" old))
 (unless (char-displayable-p new)
   (error "Not a displayable character: `%c'" new))

The current code predates the existence of
`char-displayable-p'.

(Yes, that's a different bug, but perhaps it can be
taken care of in the context of this report.  If not,
don't worry about it.)