* list/emulation/viper: Use user-error for "Viper bell"

* list/emulation/viper: Use user-error for "Viper bell"

Copyright-paperwork-exempt: yes

* lisp/emulation/viper-init.el (viper-ViperBell): New constant.
* lisp/emulation/viper-cmd.el (viper-prefix-arg-com)
(viper-forward-char, viper-backward-char, viper-goto-col)
(viper-find-char, viper-paren-match, viper-put-back, viper-Put-back)
(viper-delete-char, viper-mark-point, viper-goto-mark-subr):
* lisp/emulation/viper-ex.el (ex-delete): Use it and `user-error`.