Map file downloader adds nonexistent files to the download table

When you connect to the server, a sourcemod plugin on the server tells you to try to download a txt file with the map's description, a dds file with the map's radar, etc. However, for many maps some or all of these files don't exist. Since each request takes a fraction of a second to try to connect to the fastdl to download the nonexistent file, this can add up to a second or more of time wasted every time people connect. It also looks more unprofessional to have error messages for missing files every time someone connects.

Please check for each file's existence on the server and only add it to the download table if it is real.