- The libcurl FTP code now returns CURLE_REMOTE_FILE_NOT_FOUND error when SIZE

- The libcurl FTP code now returns CURLE_REMOTE_FILE_NOT_FOUND error when SIZE
  gets a 550 response back for the cases where a download (or NOBODY) is
  wanted. It still allows a 550 as response if the SIZE is used as part of an
  upload process (like if resuming an upload is requested and the file isn't
  there before the upload). I also modified the FTP test server and a few test
  cases accordingly to match this modified behavior.