- Improved form handling.

  + 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().