Output a warning so that we can dig into this. For now, the patch doesn't fix
the root cause. If the server returns UID 0 as valid UID number, this must be
ignored as soon as possible.
Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/336
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>