Disable maps with invalid rules

This prevents the host from starting a game with a map that would crash during rules parsing.

![invalid-map](https://cloud.githubusercontent.com/assets/167819/3006804/7c223726-de4f-11e3-824d-60aad056411b.png)

My original approach for this was to only validate custom maps (hence the fix for #4635), but preloading the rules for any map ended up being a better approach (useful for #3012).