Manage Extensions throws an erroneous fatal error - Failed Fetching List - when there are no extensions available for a particular install version

Browsing to or refreshing Manage Extensions page in 4.4 currently results in a fatal error:

Failed Fetching List - Could not retrieve a list of extensions from the CiviCRM public directory at https://civicrm.org/extdir/ver=4.4.alpha1|cms=Drupal - please contact CiviCRM team on CiviCRM forum.

This is thrown because that URL currently produces an empty page (HTTP status = 200). Since this is NOT an error (and we are trapping HTTP status errors elsewhere) we should change this to a notification with an updated message:

"No Extensions Available for $version" - "There are currently no extensions on the CiviCRM public extension directory which are compatible with your site's version ($version). If you want to install an extension which is not marked as compatible, you may be able to download and install extensions manually (depending on access to your web server)."