make savemessage() handle NO response to APPEND correctly

IMAP servers can return `NO` responses to the `APPEND` command,
e.g. here's an example response from Groupwise's IMAP server:

    NO APPEND The 1500 MB storage limit has been exceeded.

In this case, savemessage() should abort the repository sync rather
than returning UID 0 which would cause the local copy of the message
being saved to get irreversibly deleted.

Signed-off-by: Adam Spiers <offlineimap@adamspiers.org>