dbox: Log a better error if we have external attachments, but mail_attachment_dir is unset.
dbox: Log a better error if we have external attachments, but mail_attachment_dir is unset.

LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot