ensure restart save is exclusive to prevent save corruption

Torch.API/Session/GameSaveResult.cs

@@ -33,6 +33,11 @@
         /// <summary>
         /// The save operation timed out
         /// </summary>
-        TimedOut = -5
+        TimedOut = -5,
+        
+        /// <summary>
+        /// Another save operation is in progress
+        /// </summary>
+        ConcurrentSaveInProgress = -6
     }
 }

Torch.Server/TorchServer.cs

@@ -248,7 +248,7 @@ namespace Torch.Server
             {
                 if (save)
                 {
-                    var saveResult = Save().Result;
+                    var saveResult = Save(exclusive: true).Result;
                     if (saveResult is not (GameSaveResult.Success or GameSaveResult.TimedOut))
                     {
                         Log.Error("Save failed due to {Reason}. Restart aborted!", saveResult);

Torch/TorchBase.cs

@@ -330,7 +330,7 @@ namespace Torch
                 if (MyAsyncSaving.InProgress || _inProgressSaves > 0)
                 {
                     Log.Error("Failed to save game, game is already saving");
-                    return null;
+                    return Task.FromResult(GameSaveResult.ConcurrentSaveInProgress);
                 }
             }