drupal_add_js not work in form validation failed
Javascripts are lost when the form is rebuilt when using drupal_add_js(). Use $form['#attached'] instead. Also, it's better to use hook_form_FORM_ID_alter() instead of a general hook_form_alter(). hook_form_alter() will run for every form on the page. Try using the below:

function mymodule_form_news_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['#attached']['js'] = array(
    drupal_get_path('module', 'mymodule') . '/js/mymodulejsfile.js',
  );
}