append() raises an Exception, in case the IMAP server replies with 'BAD'
(but not when it responds with 'NO') but we were not catching that. Do
catch the situation and also raise an OfflineImapError at MESSAGE
severity, so that we can continue with the next message.
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>