Update Manager considers generic packages as untrusted packages
With issues like this, you can usually narrow down the problem by looking at the errors as a result of running:
sudo apt-get update && sudo apt-get upgrade
With your latest update to your question you can see a 404-error which indicates that a web-page cannot be accessed (a 404 is a standard web-response). In this case, the elementaryArt PPA is not accessible. Usually this is a server issue - which disappears if the server comes back-on line.
However, in this case, if you look on launchpad, the PPA in question does not have an oneiric repository.
As to why you get untrusted packages error - usually this is because the signature for a repository needs to be updated or is temporarily unavailable.
In your specific case, the repo-keys have been "refreshed" via the update/upgrade action.
If you at a later date receive errors such as BAD-SIG and/or NO_PUBKEY you can fix these via the linked questions below
Linked Questions:
- How do I fix the GPG error "NO_PUBKEY"?
- What is the easiest way to resolve apt-get BADSIG GPG errors?