* elmo.el (elmo-generic-folder-append-messages): Make

* elmo.el (elmo-generic-folder-append-messages): Make
fetch-strategy with `use-cache' as 'maybe. Check return value of
`elmo-message-fetch'.
(elmo-message-fetch): Return return value of
`elmo-message-fetch-with-cache-process'.
(elmo-message-fetch-with-cache-process): Return non-nil if
fetching was succeed. Load cache when fetching was failed and
fetch-strategy-use-cache is 'maybe.

* elmo-util.el (elmo-file-cache-load): New function.

* elmo-multi.el (elmo-message-use-cache-p): Remove duplicated
defun.

* elmo-archive.el (elmo-archive-message-fetch-internal): Return
non-nil if fetching was succeed.

* elmo-imap4.el (elmo-imap4-message-fetch): Likewise.

* elmo-nmz.el (elmo-map-message-fetch): Likewise.

* elmo-nntp.el (elmo-nntp-read-body): Likewise.
(elmo-message-fetch-with-cache-process): Likwise.

* elmo-pop3.el (elmo-pop3-read-body): Likewise.

* elmo-shimbun.el (elmo-map-message-fetch): Likewise.