Form validation handler for update_manager_install_form().
update_manager_install_form_submit()
function update_manager_install_form_validate($form, &$form_state) {
if (!($form_state['values']['project_url'] xor !empty($_FILES['files']['name']['project_upload']))) {
form_set_error('project_url', t('You must either provide a URL or upload an archive file to install.'));
}
}