SQL error when username is already taken

For example, if I have a user (created before remove_username was installed) with username foo@example.com and e-mail bar@example.com and I try to create a new user with email foo@example.com, it will trigger this SQL error :

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'foo@example.com-fr' for key 'user__name': INSERT INTO {users_field_data}   

The attached patch avoids this by checking if the username already exists.
It also checks the email with user_validate_name() because the username field has more strict rules than the email field.