Form validation handler for field_ui_field_overview_form().
field_ui_field_overview_form_submit()
function field_ui_field_overview_form_validate($form, &$form_state) {
_field_ui_field_overview_form_validate_add_new($form, $form_state);
_field_ui_field_overview_form_validate_add_existing($form, $form_state);
}