Added a new Lint check for invalid map cordons

This will protect both this repository and http://resource.openra.net from invalid submissions that crash the game. It is also documented at https://github.com/OpenRA/OpenRA/wiki/Mapping#file-format but no one reads manuals.
This error message could be clearer: "Invalid bounds: A 1 cell (or greater) border is required between the playable bounds and the map edge on all four sides".