[Feature]: Identify cause of invalid configuration

[PikaMug]

Describe the solution you'd like

Attempting to purchase an upgrade on 0.2.21 results in this warning:

BedWars/plugin/src/main/java/org/screamingsandals/bedwars/inventories/ShopInventory.java

Line 571 in b6d3f8e

Debug.warn("[BedWars]> Upgrade configuration is invalid.");

Please output a more descriptive warning so I can tell what's wrong.

Is your feature request related to a problem? Please describe

Here's the upgradeShop.yml I'm testing with (based on the snippet from https://docs.screamingsandals.org/BedWars/0.2.21/upgrades/):

data:
- stack: "APPLE;1;Upgrades;Upgrade your stuff."
  row: 2
  column: 3
  items:
   - price: 1
     price-type: iron
     upgrade:
      entities:
       - type: spawner
      spawner-type: iron
      add-levels: 0.2
      shop-name: "iron upgrade of the team %team%"
      notify-team: true
     stack:
      type: DIAMOND_BLOCK
      amount: 1
      display-name: "iron upgrade of the team %team%"