Usability for update error: unauthenticated sources

[Bogdan]

In order to solve the error "This requires installing packages from unauthenticated sources."
As user
I need ubuntu to show me the error cases

Given an old installed package with an key expired
When I run the Software Updater
Then I get error about "This requires installing packages from unauthenticated sources."
And info about the cause and solution

*Right now we got only the error message with 2 options:
- OK - doesn't help (just closes the window)
- Settings... - opening Software & Update settings : doesn't help because don't know the root cause.

* Searching in Internet leads to many results - many of them not good.

* Doing from command line (so not user friendly for the common user):
sudo apt-get update
W: GPG error: http://download.opensuse.org Release: The following signatures were invalid: KEYEXPIRED 1437126969

So the OS knows the cause. It just have to be integrated in gui error message (eg with Details dropbox) . Then the user will go to Settings and fix the error.