Some more logic to handle missing sub files and active file transfers when starting a round:

- server waits for transfers to finish before starting the round (up to a max 20 seconds, can be skipped by the host)
- clients enable the spectate button when the round starts (in case they fail to start the round due to a missing sub file or an error)
- clients notify the server if a transfer is cancelled

+ FileReceivers can't be instantiated if a server is running

Subsurface/Source/Networking/FileTransfer/FileSender.cs

@@ -109,6 +109,11 @@ namespace Barotrauma.Networking
 
         private NetPeer peer;
 
+        public List<FileTransferOut> ActiveTransfers
+        {
+            get { return activeTransfers; }
+        }
+
         public FileSender(NetworkMember networkMember)
         {
             peer = networkMember.netPeer;