Ask users for username and password if download fails

[serialseb]

Two type of passwords may be needed by the shell. The first is the proxy password in case of BASIC/DIGEST kind of proxy, the second is the proxy to access the actual server (in case that server requires authetnication).

Right now -ProxyUsername and -ProxyPassword covers the first scneario, we don't cover the second one, and the shell should be made interactive so we query the user rather than error out.