spearmint: Fix downloading pk3s from dedicated server

Client has fs_serverReferencedPaks set, dedicated server does not.
spearmint: Allow downloading standalone games

When connecting, the client tries to change fs_game before downloading pk3s.
So if client cannot switch fs_game, switch back to previous game and download the files.
Client changes fs_game and restarts filesystem after downloads are done.
Fix sending pk3 download error messages to clients

Disabled in e52a492f615e1245d6d22a84d2cc100c6d2f9f88.
Merge remote-tracking branch 'ioquake3/master'