Postmortem doesn't (to me at least) imply failure. Specifically for games it's just a critical analysis of what worked and what didn't on a project for the purposes of learning for the next one.

I don't know if the phrase was originated by Game Developer Magazine, but it certainly popularised it's use within the games industry. The GD Mag postmortems are generally exceptionally candid and a great series to learn things from. See http://www.gamasutra.com/features/postmortem/ and the general archives at http://www.gdcvault.com/gdmag (I'm not sure if the website has all the postmortems that featured in the, sadly defunct, printed magazine).