Improve equip fail messages.

Message when trying to equip regular items to slots.
Mention ID slot specifically.

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

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks.Dataflow;
@@ -158,6 +158,11 @@ namespace Content.Server.GameObjects
                 reason = controllerReason ?? reason;
             }
 
+            if (!pass && reason == null)
+            {
+                reason = Loc.GetString("You can't equip this!");
+            }
+
             return pass && SlotContainers[slot].CanInsert(item.Owner);
         }