Allow to delete all prepared mails even when the serialized message is missing

The deleteAll() method iterates over all mail statuses (DB) and then for each of them, tries to remove the message on the FS. However, if the message is missing this will stop the deletion and error out. We need to bulletproof this and instead report some warnings if the message on the FS fail to be removed and even don't generate any warning when there's no message on the FS.