Threads: Unable to view Root message or Incomplete threads

[daniellekirkwood]

Your use case

There's a variety of reasons a Thread may not be viewable to the user. In those instances we should display an "error" message. This allows the user access to the Thread but explains why they may be missing some messages

@janogarcia shared this figma file:
https://www.figma.com/file/T309ztx0sNyOOK6NKVLHsK/Threads?node-id=3267%3A338027

Related: element-hq/element-meta#1925