bug#34093: auto-save prevents quitting emacs on read-only filesystem

[Leonard Lausen]

Recently my filesystem had to be emergency remounted as read-only. I
observed that I could not kill-emacs anymore. The following showed up in
my log upon trying to quit emacs:

```
company-statistics--save: Opening output file: Read-only file system, 
/home/leonard/.emacs.d/.cache/company-statistics-cache.el
Error running timer ‘savehist-autosave’: (file-error "Opening output file" 
"Read-only file system" "/home/leonard/.emacs.d/.cache/savehist")
```

This prevented quitting emacs.

I am not sure if `company-statistics--save` prevented the shutdown, or
just warned about the read-only file system. Possibly the behavior of
`company-statistics--save` is sane and only `savehist-autosave` is at
fault. Maybe both are at fault?

The expected behavior is that emacs may warn about or ignore that it
cannot write the savehist file. Calling kill-emacs should still quit
emacs.

The company-mode bug report is at
https://github.com/company-mode/company-mode/issues/860

This is with GNU Emacs 26.1 and the following init.el
https://github.com/leezu/dotfiles/blob/master/emacs/.spacemacs.d/init.el