Fix NPE in Gambleshops and add missing messages for gamble shops.

src/main/java/com/alttd/playershops/shop/PlayerShop.java

@@ -308,7 +308,7 @@ public class PlayerShop {
     private TransactionError executeGambleTransaction(Player player) {
         ItemStack itemStack = getRandomItem();
         if (itemStack == null)
-            return null;
+            return TransactionError.INSUFFICIENT_FUNDS_SHOP;
 
         itemStack = itemStack.clone();
         double price = getPrice();