Reworked thread managment for downloads.  The same worker threads
are now re-used, pulling from a work queue.
Renamed VOSpaceErrorHandler to DownloadErrorHandler.
Retry-download feature now working end-to-end.  However, if one's 
network connection goes down it creates a flood of popups which should
be remedied.
Don't use daemonized threads because weird error messages get 
dumped if you shut down the application while mid-download.
Only allow one pop-up to retry downloads at a time.  Selecting retry 
will cause all failed downloads to be requeued.