circmgr: When planning, only keep one error; log them all.

        // Okay, we need to launch circuits here.

        let parallelism = std::cmp::max(1, self.builder.launch_parallelism(usage));

        let mut plans = Vec::new();

        let mut errs = Vec::new();

        let mut last_err = None;

        for _ in 0..parallelism {

            match self.plan_by_usage(dir, usage) {

                Ok((pending, plan)) => {

                    list.add_pending_circ(pending);

                    plans.push(plan);

                }

                Err(e) => errs.push(e),

                Err(e) => {

                    debug!("Unable to make a plan for {:?}: {}", usage, e);

                    last_err = Some(e);

                }

            }

        }

        if !plans.is_empty() {

            Ok(Action::Build(plans))

        } else if let Some(last_err) = errs.pop() {

        } else if let Some(last_err) = last_err {

            Err(last_err)

        } else {

            // we didn't even try to plan anything!