Need error messages for failed attempts to join games

When joining a game fails, it would be nice to show an error message explaining why. Examples include:

1. The network connection failed.
2. The game wasn't found.
3. The game was found, but it was the wrong game (mismatched signature)
4. The game has already started.

No files or tests are named. Start by tracing the game-joining flow and how its network failures are currently surfaced. Done means failed joins distinguish connection failure, missing games, mismatched signatures, and games that have already started with user-visible messages.

Written by the indexing model from the issue text.