Threads: Improve handling of encryption errors across all clients

Reevaluate how to display and handle encryption errors in root messages (thread list) and summaries (main timeline, thread list, search, mentions...).

**Challenges**

- How can we solve an action to help the user recover from the error?
- How can we show a useful error message that doesn't get truncated.

No files, tests, or client entry points are named. Begin by mapping encryption errors in root messages and summaries across the thread list, main timeline, search, and mentions; done requires an agreed recovery action and a useful error message that is not truncated across all clients.

Written by the indexing model from the issue text.