spa-js/app/Http/Middleware/Authenticate.php

<?php

namespace App\Http\Middleware;

use App\Exceptions\ApiException;
use Illuminate\Auth\Middleware\Authenticate as Middleware;

class Authenticate extends Middleware
{
    protected function unauthenticated($request, array $guards)
    {
        throw new ApiException(403, "Login failed");
    }
}