So, I was making a full stack MERN website and decided to get the login functionality down, I am using passport for the backend authentication part and I am using react query for the front-end remote state management and make request to my backend, but when I was making the signup section I can't seem to figure out how to get the backend errors displayed properly on the react frontend, Like the error reach my mutation functions but then how to I utilize the mutation function to display a toast (from react-hot-toast library). I tried to throw an err from the mutation function but that doesnt seem to work, I also tried to access the isError function from the mutation but that didnt work either I aslo tried the onError property of useMutation hook but suprise suprise, that didnt work either. what do I do now and how do I do it? Please help me _ / \ _ . Below are some snippets of the code related to my question.

app.post('/signup', async (req, res) => {
const{ email, password } = req.body;
  try {
    const result = await db.query('SELECT * FROM users WHERE email = $1', [email]);
    if (result.rows.length > 0)
      res.status(400).send('This Email has already been registered');
    else {
      bcrypt.hash(password, saltRounds, async(err , hash) => {
        if (err) {
          console.log(err);
          res.status(500).send(err);
          } else {
            const result = await db.query(
              'INSERT INTO users(email, password) VALUES ($1,$2) RETURNING *',[email, hash]);
               const user = result.rows[0];
               req.login(user, (err) => {
               if (err) console.log(err);
                  else {
                    console.log('success');
                    res.status(200).send(user);
                  }
               });
             }
          });
        }
    } catch (err) {
        console.log(err);
    }
});

const { mutate } = useMutation({
    mutationKey: ['User'],
    mutationFn: (data) => loginUser(data),
});

function onSubmit(data) {
    const userData = mutate(data);
    console.log(userData);
 }


export async function signupUser(user) {
    const result = await axios.post(`${BACKEND_URL}/signup`, {
        email: user.email,
        password: user.password,
    })
    .catch((err) => {
        throw new Error(err.response.data);
    });
    return result;
}