Better label messages that aren't encrypted for any device

If a user has logged out of all of their active sessions, then they'll never be able to decrypt messages sent to them while they were offline. Instead of labelling messages generically as 'unable to decrypt [request keys from other devices]' we should label them more explicitly.

The issue names no file, test, or entry point. Start by locating the client code that renders the generic unable-to-decrypt message and its related tests; done means messages that cannot be decrypted because all active sessions were logged out receive a more explicit label.

Written by the indexing model from the issue text.