Assist: Fail move if no moves have been found

Scripts/Interfaces/MoveTurnChoice.astypedef

@@ -3,5 +3,7 @@ type MoveTurnChoice {
     Pokemon@ User { get const; };
     LearnedMove@ Move { get const; };
     int8 Priority { get const; };
+    bool HasFailed { get const; };
+    void Fail();
     BaseTurnChoice@ opImplCast();
 }

Scripts/Moves/Assist.as

@@ -20,7 +20,7 @@ namespace Gen7 {
                 }
             } 
             if (possibleMoves.length == 0){
-                // TODO: Log failure.
+                move.Fail();
                 return;
             }
             auto i = battle.Random.Get(possibleMoves.length);