Comment 36 for bug 24250

Thanks Serge
I tried return True as described in comment 28
https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/24250/comments/28

He is right, that this routine url_downloadable() is the cause of the issue.
By skipping straight over it, all is well.

Until there is a patch, I believe this is a solution to getting your boxes upgraded behind a proxy or apt-cacher.

John