"Is it Pilot error if you hit an unseen cable"?
It's not anybody else's is it??. (Except possibly in the case of the NZ incident when someone else took action that moved the cable into the disc). Yes, there may be mitigation / excuses i.e "It was difficult to see" or "It was not marked on the map" but ultimately the pilot is responsible for the safety of the aircraft - if you're going down to low-level then you know that wires are a risk and must avoid them - end of story.
I speak here as someone who has twice come very close to clattering "invisible" wires in landing sites. Fortunately i saw them at the very last minute and managed to miss them - it would have been no-one's fault but my own if i had hit them.
OH