I would look at making a custom auth driver for this.
You could then use the built in auth api with your custom driver to handle the authentication.
https://laravel.com/docs/5.2/authentication#adding-custom-guards
This should let you treat the login just like a regular login in Laravel, with redirects, errors, etc. but using your special authentication method for the backend password/username checking.
Sign in to participate in this thread!
Laravel
Laravel News
Laracasts
Laravel Podcast