Refactor inventory listing filters to return error messages for invalid user, location, and item IDs

.gitignore

@@ -1 +1,2 @@
 __pycache__/
+.venv/

routes.py

@@ -158,14 +158,17 @@ def list_inventory():
         if column is not None:
             filter_name = None
             if filter_by == 'user':
-                user = db.session.query(User).filter(User.id == id).first()
-                filter_name = user.full_name if user else "Unknown User"
+                if not (user := db.session.query(User).filter(User.id == id).first()):
+                    return "Invalid User ID", 400
+                filter_name = user.full_name
             elif filter_by == 'location':
-                room = db.session.query(Room).filter(Room.id == id).first()
-                filter_name = room.full_name if room else "Unknown Location"
+                if not (room := db.session.query(Room).filter(Room.id == id).first()):
+                    return "Invalid Location ID", 400
+                filter_name = room.full_name
             else:
-                item = db.session.query(Item).filter(Item.id == id).first()
-                filter_name = item.description if item else "Unknown Item"
+                if not (item := db.session.query(Item).filter(Item.id == id).first()):
+                    return "Invalid Type ID", 400
+                filter_name = item.description
 
             query = query.filter(column == id)
         else: