Returns whether a menu name already exists.
menu_edit_menu()
function menu_edit_menu_name_exists($value) {
$custom_exists = entity_load('menu', $value);
// 'menu-' is added to the menu name to avoid name-space conflicts.
$value = 'menu-' . $value;
$link_exists = Drupal::entityQuery('menu_link')
->condition('menu_name', $value)
->range(0, 1)
->count()
->execute();
return $custom_exists || $link_exists;
}