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
     }
 }