Add error popup message on equip failure (#955)

Content.Server/GameObjects/Components/GUI/IInventoryController.cs

@@ -14,7 +14,14 @@ namespace Content.Server.GameObjects
         /// <param name="slot">The slot to be equipped into.</param>
         /// <param name="entity">The entity to equip.</param>
         /// <param name="flagsCheck">Whether the entity passes default slot masks & flags checks.</param>
+        /// <param name="reason">The translated reason why the item cannot be equiped, if this function returns false. Can be null.</param>
         /// <returns>True if the entity can be equipped, false otherwise</returns>
-        bool CanEquip(Slots slot, IEntity entity, bool flagsCheck) => flagsCheck;
+        bool CanEquip(Slots slot, IEntity entity, bool flagsCheck, out string reason)
+        {
+            reason = null;
+            return flagsCheck;
+        }
+
+        bool CanEquip(Slots slot, IEntity entity, bool flagsCheck) => CanEquip(slot, entity, flagsCheck, out var _);
     }
 }