Show login errors in Community Auth

Hi.

I have setup correctly community auth in my project, and all works fine.
However, I need help with some views that I am developing.

My problem is that I want show login errors individually in the login page.

In example controller, when isset($login_error_mesg) in the login view, it shows a hardcoded message.
That I want is that for example, if username is required, when that input is empty it shows a message like "User is required".
How can I get it?
I have used in other time CI library form validation, and when it was any error I showed with form_error() or with validation_errors(), but now I don't know how to do with Community Auth.

In the file login.php (config/form_validation/examples/login.php) they are stablished all custom rules, but when this rules broken, I don't know how I can show specific error in the view.

Thanks.