Scrub channel target information in errors

Part of #556 (closed).

I went through the errors I could find and addressed them. The LoggedChanTarget (#556 (comment 2855921)) seems to sit nicely, and in fact I was able to do it generically (as mooted in #556 (comment 2855810)).

Because the new generic BoxSensitive type is rather semantically bland, I chose to use type aliases or renamed imports in a couple of places.

@nickm, I think it might be worth you checking that I didn't miss anything. In particular, I think I might easily have missed whole error types somewhere.