org-agenda.el: Use `user-error' instead of `error' in some places

org-agenda.el: Use `user-error' instead of `error' in some places

* org-agenda.el (org-agenda-undo, org-agenda)
(org-agenda-append-agenda)
(org-agenda-get-restriction-and-command, org-agenda-write)
(org-agenda-clock-cancel)
(org-agenda-diary-entry-in-org-file, org-agenda-diary-entry)
(org-agenda-execute-calendar-command)
(org-agenda-goto-calendar, org-agenda-convert-date)
(org-agenda-bulk-mark, org-agenda-bulk-action)
(org-agenda-show-the-flagging-note): Use `user-error' instead
of `error'.