* after downloading a download, set the downloads.file_status to a non-"missing" value to prevent a missing download from being redownloaded over and over again;
* rename _enqueue_broken_downloads_if_idle() to _add_missing_downloads() and move it to provide_downloads() to make it less of a procedure and more of a function;
* limit the missing downloads by the site just like the _add_pending_downloads() does