Gracefully handle single-room mode when room is not available to user

There are several scenarios that can result in the user not being able to access the configured room, in single-room mode:

1. Room doesn't exist
2. User can't access room
- Because room is private
- Because room is not under a Space the user has access to
- etc
3. Configured room alias doesn't resolve to a room

We should gracefully handle this case, by either:

1. Showing an error screen
2. Falling back to non-single-room mode

Trace the single-room mode entry point and the handling of the configured room or room alias, then reproduce cases where the room is missing or inaccessible. Confirm with maintainers whether the expected result is an error screen or fallback to non-single-room mode; done means the chosen behavior covers all listed scenarios.

Written by the indexing model from the issue text.