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

[psrpinto]

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