Throw, test, and document better errors.

[benrosen]

Files to consider:

• pixelchessResolveCreateConnectionMutation
• pixelchessResolveCreateGameMutation
• pixelchessResolveSetPlayerMutation
• GameManager
• createGame
• joinGame
• createGameSubscription
• pixelchessResolveUpdateStatusMutation
• pixelchessQueryPlayerByGameId

[benrosen]

Use the common-errors package, which should be installed as a dev dependency.