bug#29412: 27.0.50; dired-toggle-read-only should (at some point!) check that the direcory is writeable

[Robert Marshall]

In a shell

mkdir dired-err
touch dired-err/xx
chmod 555 dired-err

emacs -Q
M-x dired  dired-err
C-x C-q
edit the xx file name to taste
C-c C-c (finish the edit)
You exit the filename editing mode with an error (not surprising) but
all your edits are lost. It would be more helpful if either C-x C-q gave
you a warning or the finish of the edit spotted that nothing was going
to work and complained or that it didn't exit the mode (though I realise
that there may be cases where some edits may have worked e.g. where the
dired-err subdir has been inserted in another dired session before
editing the file names)

In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 2.24.25)
 of 2017-11-14 built on ct-lt-579
Repository revision: 13248f7444630508cfc3b78a07e8d96613af11c8
Windowing system distributor 'The X.Org Foundation', version 11.0.11604000
System Description:     Debian GNU/Linux 8.9 (jessie)

Recent messages:
Saving file /home/robertmarshall/.newsrc.eld...
Wrote /home/robertmarshall/.newsrc.eld
Saving /home/robertmarshall/.newsrc.eld...done
Directory has changed on disk; type g to update Dired
Press C-c C-c when finished or C-c ESC to abort changes
1 rename actions failed--type ? for details
Auto-saving...
Quit
Type C-x 1 to delete the help window.
Scanning for dabbrevs...done [2 times]

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND DBUS GSETTINGS NOTIFY GNUTLS LIBXML2
FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS GTK2 X11

Important settings:
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Info

Minor modes in effect:
  magit-auto-revert-mode: t
  global-git-commit-mode: t
  async-bytecomp-package-mode: t
  shell-dirtrack-mode: t
  gnus-desktop-notify-mode: t
  diff-auto-refine-mode: t
  desktop-save-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows: