bug#19823: 25.0.50; Inferior-python-mode buffer can't be killed if process is dead

[Dima Kogan <dima@secretsauce.net>]

Hi. Currently emacs requires the inferior python process to exist to be
able to kill that buffer. If the inferior process exits earlier for any
reason, it's impossible to kill the buffer. Recipe:

1. emacs -Q
2. M-x python
3. exit()       [ this exits the session, killing the inferior process ]
4. C-x k enter  [ Error! "No inferior Python process running" ]


This happens because a cleanup handler fails. It needs to explicitly
ignore this particular failure. Wrapping the contents of
python-shell-font-lock-kill-buffer() into (ignore-errors ....) is one
way to fix this:

(defun python-shell-font-lock-kill-buffer ()
  "Kill the font-lock buffer safely."
  (ignore-errors
    (python-shell-with-shell-buffer
      (when (and python-shell--font-lock-buffer
                 (buffer-live-p python-shell--font-lock-buffer))
        (kill-buffer python-shell--font-lock-buffer)
        (when (derived-mode-p 'inferior-python-mode)
          (setq python-shell--font-lock-buffer nil))))))

It should probably be more specific than a blanket ignore-errors, but
the maintainers should decide what they want here.