After ongoing Signal session is removed locally we discard incoming messages from that session

[davidchiles]

Steps to reproduce:

1. Two chatsecure clients talking over OMEMO.
2. On device 1 remove all the other buddies devices (including device 2)
3. Send messages from device 2. Error messages are suppressed and fail to decrypt.
4. device 1 starts sending OMEMO messages and creates a new session.
5. Everything works great.

Fix:
We need some way of refreshing the session or at the least letting the user know that we can't decrypt incoming messages.