* lisp/dired.el (dired-get-marked-files): Add new optional arg 'error'.

* lisp/dired.el (dired-get-marked-files): Add new optional arg 'error'.

Call user-error when it's non-nil and result is empty (bug#30285).

* lisp/dired-aux.el (dired-do-chxxx, dired-do-chmod)
(dired-do-print, dired-do-async-shell-command)
(dired-do-shell-command, dired-do-compress-to)
(dired-mark-confirm, dired-do-create-files)
(dired-do-isearch, dired-do-isearch-regexp)
(dired-do-query-replace-regexp, dired-do-find-regexp):
* lisp/dired-x.el (dired-do-find-marked-files):
Set arg 'error' of dired-get-marked-files call to t.