add a sensible error when trying to save a map using a bogus tileset

OpenRA.Game/Map.cs

@@ -375,7 +375,12 @@ namespace OpenRA
 				// File header consists of a version byte, followed by 2 ushorts for width and height
 				writer.Write(TileFormat);
 				writer.Write((ushort)MapSize.X);
-				writer.Write((ushort)MapSize.Y);
+				writer.Write((ushort)MapSize.Y);
+
+				if (!OpenRA.Rules.TileSets.ContainsKey(Tileset))
+					throw new InvalidOperationException(
+						"Tileset used by the map ({0}) does not exist in this mod. Valid tilesets are: {1}"
+						.F(Tileset, string.Join(",", OpenRA.Rules.TileSets.Keys.ToArray())));
 
 				// Tile data
 				for (int i = 0; i < MapSize.X; i++)