Is it pilot error if you hit an unseen cable?
Yup, I think so. The pilot should have considered that it 'might' be there, and flown, briefed or acted accordingly. It's a harsh way to look at it, but the only pre-strike approach that will prevent strikes.
It's the same as on a motorbike: it doesn't matter whether the person who pulls out in front of you is an idiot or not, the rider is the one who suffers in the accident. So it's 'my' fault that I didn't consider what the idiot might do, or read the road ahead for possible hazards.
I'm not knocking the pilot's expertise, just making an observation about how one could approach the issue.
(Not-so-young) Lafite