User Registration Form error message customisation
You can do this using Craft's static translation support.

For example, if your site is using en_us for its primary locale, then add a craft/translations/en_us.php file that has this in it:

&lt;?php

return array(
    '{attribute} "{value}" has already been taken.' =&gt; '{attribute} "{value}" has already been taken. Login here instead.',
);