+ Introduced two new functions:
1. form_set_error($name, $message): files an error against the form
element with the specified $name.
2. form_has_errors(): returns true if errors has been filed against
form elements.
+ Updated the form handling:
1. The form_ functions will add 'class="error"' when a form field
has been found to be erroneous.
2. The error message is passed to theme_form_element() when the
particular form field has been found to be erroneous.
+ I updated the user and profile module to take advantage of these new
functions.
+ IMPORTANT: the _user() hook changed. The 'validate' case should no
longer retun an error message when something goes wrong but should
set it with form_set_error().