Get an error message when email is already in use

I created a very simple registration form with only email. Plugin uses the same email as username and account’s email and WP registration system sends an email with the new password. So far, so good. But when I send an existing email address, the form only shows the success message, as set in form parameters.

Visitor can’t register, as expected, but he’s unaware the registration process failed. How do I get the respective error message ’email is already in use’?

I guess the same could apply if username is repeated.