ftp handling should be more robust

[Francesco Potorti` <F.Potorti@cnuce.cnr.it>]

This is an example of a recent download of mine:

Get ftp://ftp.de.debian.org slink/main perl [4118k]
Get ftp://ftp.de.debian.org slink/main libhdf4g [370k]
Get ftp://ftp.de.debian.org slink/main svgalib-dummyg1 [17.9k]
Error ftp://ftp.de.debian.org slink/main svgalib-dummyg1
 425 Can't open passive connection: Invalid argument.
Get ftp://ftp.de.debian.org slink/main geg [30.8k]
Get ftp://ftp.de.debian.org slink/main gnuplot [1032k]

The error  you see  is a  transient one, probably  due to  server overload.
Calling apt  again downloads the file  without problems.  I  think that apt
should be more clear in its error  message, and also that it should be more
persistent in the  face of ftp errors, that is, it  should retry the failed
downloads, possibly after having tried the other ones.

Thanks.