Running updates via sudo apt-get update, I've been recently having these types of errors that do not affect the system performance.
W: GPG error: http://download.opensuse.org/repositories/home:/stevenpusser/xUbuntu_17.04 Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 0FAD31CA8719FCE4 E: Failed to fetch http://ppa.launchpad.net/midori/ppa/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found E: Some index files failed to download. They have been ignored, or old ones used instead. val@val-ThinkPad-X200s:~$
I would appreciate if someone told me how to fix this particular problem, and how to address similar problems in the future.
Thank you and happy New Year to everyone.