Handle cancelling out of game selection

Core/GameInstanceManager.cs

@@ -206,7 +206,7 @@ public GameInstance AddInstance(string path, string name, IUser user)
         {
             var game = DetermineGame(new DirectoryInfo(path), user);
             if (game == null)
-                throw new NotKSPDirKraken(path);
+                return null;
 
             return AddInstance(new GameInstance(game, path, name, user));
         }
@@ -608,14 +608,15 @@ public static bool IsGameInstanceDir(DirectoryInfo path)
         /// </summary>
         /// <param name="path">A DirectoryInfo of the path to check</param>
         /// <param name="user">IUser object for interaction</param>
-        /// <returns>An instance of the matching game, or null if none could be found</returns>
+        /// <returns>An instance of the matching game or null if the user cancelled</returns>
+        /// <exception cref="NotKSPDirKraken">Thrown when no games found</exception>
         public IGame DetermineGame(DirectoryInfo path, IUser user)
         {
             var matchingGames = knownGames.Where(g => g.GameInFolder(path)).ToList();
             switch (matchingGames.Count)
             {
                 case 0:
-                    return null;
+                    throw new NotKSPDirKraken(path.FullName);
 
                 case 1:
                     return matchingGames.First();

GUI/Dialogs/CloneFakeGameDialog.cs

@@ -146,13 +146,14 @@ private async void buttonOK_Click(object sender, EventArgs e)
                 try
                 {
                     IGame guessedGame = manager.DetermineGame(new DirectoryInfo(existingPath), user);
+                    if (guessedGame == null)
+                    {
+                        // User cancelled, let them try again
+                        reactivateDialog();
+                        return;
+                    }
                     await Task.Run(() =>
                     {
-                        if (guessedGame == null)
-                        {
-                            throw new NotKSPDirKraken(existingPath);
-                        }
-
                         GameInstance instanceToClone = new GameInstance(
                             guessedGame,
                             existingPath,