Signature warning should include full repository name/path

Most people nowadays can collect quite a few APT repositories hosted on the same server (ppa.launchpad.net, for instance). In case the signature check on one of these repositories fails, apt-get just prints this:

W: GPG error: http://ppa.launchpad.net saucy Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY <blah>

Unfortunately, this informs the user only that one of the repositories on ppa.launchpad.net has a missing key. But not which one. It could be any of them. Tracking down the specific signature (<blah> in my example) is difficult so the only practical option is to disable the repositories one by one until the warning disappears. Which is not only annoying to the user but also puts unnecessary load on the servers which get repeatedly hit by 'apt-get update'.

I therefore suggest to change the warning to include the full path of the repository. Example:

W: GPG error: http://ppa.launchpad.net/ubuntu-wine/ppa/ubuntu saucy Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY <blah>

This would inform the user which repository needs a key. Other options (such as printing the repository's name are also valid, of course).

PS: I am aware that apt-add-repository automatically adds the key but this magic only works on Launchpad whereas the problem is one in general. Furthermore, you don't always get to use apt-add-repository for various reasons.