客户端只接收您使用res.send()或next(err)执行的操作，然后next(err)将调用res.send()。承诺只在服务器本地执行，而不会被发送回客户端。
在你的原始代码中，我建议你只使用基于承诺的异步操作，然后你可以在你的代码中使用throw，有一个地方来捕捉所有的错误，然后从那里将错误发送回客户端。

class ServerError extends Error {
    constructor(msg, status) {
        super(msg)
        this.status = status;
    }
}

app.post('/signup', (req, res) => {
    try {
        const email = req.body.email
        const plainTextPassword = req.body.password;

        //check if user already exists
        const existingUser = await User.find({ email: email });
        //account doesnt exist
        if (existingUser.length !== 0) {
            throw new ServerError('Account already exist', 403);
        }
        const hash = await bcrypt.hash(plainTextPassword, saltRounds);
        const user = new User({
            email: email,
            password: hash
        });
        const result = await user.save();
        res.send(result);
    } catch (e) {
        if (!e.status) e.status = 500;
        console.log(e);
        res.status(e.status).send({err: e.message});
    }
});

然后，在使用fetch()的客户机代码中，需要检查result.ok，查看是否返回了2xx状态。fetch()仅在与目标主机的网络连接失败时才会拒绝。如果连接成功，即使它返回错误状态，fetch()承诺将得到解决。您必须检查result.ok以查看是否得到了2xx状态。

//reduxSlice.js
export const signup = createAsyncThunk(
    'userAuth/signup',
    async (payload, thunkAPI) => {
        const { email, password } = payload
        try {
            const result = await fetch(
                signupPath, {
                mode: 'cors',
                credentials: 'include',
                method: "post",
                body: JSON.stringify({ email, password }),
                headers: {
                    'Content-Type': 'application/json'
                }
            });
            // check to see if we got a 2xx status success
            if (!result.ok) {
                throw new Error(`signup failed: ${response.status}`);
            }
            return result.json()
        } catch (error) {
            console.log(error) //this line executes
        }

    }
)