Flickr Widget: verify feed in widget settings, and display error message when invalid

When setting up a new Flickr Widget, it would be nice if we checked the RSS feed in the widget settings right after the URL is entered into the field, and displayed an error message in the settings if the feed is empty or if the URL is wrong.

The issue does not name files or tests; start by locating the Flickr Widget implementation and its widget settings form. Trace how the RSS URL is handled, then verify that empty or invalid feeds produce an error in the settings after the URL is entered.

Written by the indexing model from the issue text.