silent fetch failure if proxy mis-configured

Binary package hint: update-manager

During a Dapper->Gutsy upgrade, if /etc/apt/apt.conf contains:

  Acquire::http::Proxy "http://localhost:3128/";

And there is no proxy listening, the upgrade silently fails -- it appears that the upgrade fetched the files but then could not extract them (the actual case is that it never fetched them):

  # do-release-upgrade -d
  Checking for a new ubuntu release
  Failed Upgrade tool signature
  Failed Upgrade tool
  Done downloading
  extracting 'hardy.tar.gz'
  Failed to extract
  Extracting the upgrade failed. There may be a problem with the network or with the server.

strace shows the http method failing:

  [pid 29230] write(1, "400 URI Failure\nURI: http://archive.ubuntu.com/ubuntu/dists/hardy/main/dist-upgrader-all/current/hardy.tar.gz\nMessage: Could not connect to localhost:3128 (127.0.0.1). - connect (111 Connection refused) \nFailReason: ConnectionRefused\nTransient-Failure: true\n\n", 259) = 259