fix: return "message" instead of "error" so it is handled better at the client.

lnbits/extensions/usermanager/views_api.py

@@ -69,7 +69,7 @@ async def api_usermanager_users_delete(user_id):
 async def api_usermanager_activate_extension():
     user = get_user(g.data["userid"])
     if not user:
-        return jsonify({"error": "no such user"}), HTTPStatus.NO_CONTENT
+        return jsonify({"message": "no such user"}), HTTPStatus.NOT_FOUND
     update_user_extension(user_id=g.data["userid"], extension=g.data["extension"], active=g.data["active"])
     return jsonify({"extension": "updated"}), HTTPStatus.CREATED