Exception when directory doesn't exist upon map init

https://github.com/kaczy93/centredsharp/blob/e4563d5ca50254f4358a391c9ca17a46ae73adb3/Server/Map/Landscape.cs#L101C32-L101C32

When the directory doesn't exist it will crash with a DirectoryNotFoundException

> Map file not found, do you want to create it? [y/n]
> y
> System.IO.DirectoryNotFoundException: Could not find a part of the path