Improve end-user error message when they attempt to register as a new user with an email that belongs to an existing user

The error message currently provided is not too helpful, and probably is not recommended the generally appropriate action:

"This email %1 is already registered. Please select another email."

As discussed here on the forum:
http://forum.civicrm.org/index.php/topic,26392.0.html

... modifying the message - modeled after Drupal's message for the same condition:

"The email address joe@example.org is already registered. <a href=\"/user/password">Have you forgotten your password?</a>"