Update UserRegistrationWebformHandler.php. Add error message to configuration...
requested to merge issue/webform_user_registration-3294652:3294652-custom-username-taken into 8.x-1.x
Update UserRegistrationWebformHandler.php. Add error message to configuration of Handler, default value to match core, and create account code to check for existing user name, if exist escaping out of account creation and setting custom error message.
Closes #3294652