Digester.py is called from cron/senddigests and Web CGI
and is responsible in processing the digest.mbox file
to generate single digest message. As the process may
contain complicated task, it may fail by unknown reason.
With this patch, we put the digest.mbox file which
caused a error off by renaming as digest.mbox.(unixtime).
Without this patch, the cause of the error remeins in
the digest.mbox file and may prevent from being