Failure to read a feed produces ugly PHP error instead of drupal message

If for some reason the call to feedmanager_query_feed() fails, usually because SimplePie has likewise failed, to read a valid feed, it returns a value of FALSE. But the return value is not checked and in the following lines, is used immediately to call method get_feed_title() which throws a PHP error.

I suggest the following check:

  if ($feed === FALSE) {
    // simplepie failed to return a valid feed.
    return 'admin/aggregator';
  }