(message-send-mail-function): Fix error convention.

(message-send-mail-function): Fix error convention.
(message-mailer-swallows-blank-line, message-send-mail-with-sendmail)
(message-widen-reply, message-send-mail, message-talkative-question)
(message-with-reply-buffer, message-generate-new-buffer-clone-locals)
(message-clone-locals, message-send-news): Use with-current-buffer.
(message-insert-or-toggle-importance): Remove unused var `valid'.
(message-make-references): Remove unused var `new-references'.
(message-make-mail-followup-to): Remove unused var `subscribed-lists'.