Error related to notifications/updates

Hi all,

I installed Ubuntu a few months ago, but have started using it more and more recently. I noticed when I tued my computer on today that the notifications symbol had some error, shown in the attached picture (with the error message)

I have tried clicking updates/etc. and the one probably-useful error message I have gotten is:

W:Failed to fetch http://ppa.launchpad.net/claudiocn/s...amd64/Packages 404 Not Found
, W:Failed to fetch http://ppa.launchpad.net/claudiocn/s...-i386/Packages 404 Not Found
, E:Some index files failed to download. They have been ignored, or old ones used instead.

Can anyone explain what this means and how to fix this?

Thank you very much!