Fix to prevent betas from joining ended games

#### What's this PR do?
Previously betas could enter a game after the alpha's decided to play solo instead. This would break the game flow and cause bugs for the users as they tried to progress through the game again.

#### How should this be manually tested?
There's probably a handful of bugs that this caused, but the one scenario that we saw reported and the one that I tested:
1. Alpha creates a game A inviting 3 other betas
2. Alpha decides to play solo, effectively creating a game B and closing game A
3. Beta accepts the invite and tries to join.

Result: Alpha continues to play solo uninterrupted. Beta receives the "unable to join game in progress" message.