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/FileReceiver.cs

@@ -154,6 +154,11 @@ namespace Barotrauma.Networking
 
         public FileReceiver(string downloadFolder)
         {
+            if (GameMain.Server != null)
+            {
+                throw new InvalidOperationException("Creating a file receiver is not allowed when a server is running.");
+            }
+
             activeTransfers = new List<FileTransferIn>();
 
             this.downloadFolder = downloadFolder;
@@ -161,6 +166,11 @@ namespace Barotrauma.Networking
         
         public void ReadMessage(NetIncomingMessage inc)
         {
+            if (GameMain.Server != null)
+            {
+                throw new InvalidOperationException("Receiving files when a server is running is not allowed");
+            }
+
             System.Diagnostics.Debug.Assert(!activeTransfers.Any(t => 
                 t.Status == FileTransferStatus.Error ||
                 t.Status == FileTransferStatus.Canceled ||