Solve two annoying problems with ftp URLs for Jack: when repeated

Solve two annoying problems with ftp URLs for Jack: when repeated
retrieving files from the same host and directory, you had to close
the previous instance before opening a new one; and retrieving a
non-existent file would return an empty file.  (The latter fix relies
on maybe an undocumented property of NLST -- NLST of a file returns
just that file, while NLST of a non-existent file returns nothing.  A
side effect, unfortunately, seems to be that now ftp-retrieving an
*empty* directory may fail.  Ah well.)