Fix assert unreachable, remove unused variables (#13753)

Small code cleanup

packages/twenty-server/src/engine/core-modules/webhook/utils/webhook-graphql-api-exception-handler.util.ts

@@ -1,3 +1,5 @@
+import { assertUnreachable } from 'twenty-shared/utils';
+
 import {
   NotFoundError,
   UserInputError,
@@ -17,9 +19,7 @@ export const webhookGraphqlApiExceptionHandler = (error: Error) => {
           userFriendlyMessage: error.userFriendlyMessage,
         });
       default: {
-        const _exhaustiveCheck: never = error.code;
-
-        throw error;
+        return assertUnreachable(error.code);
       }
     }
   }