Improve error reporting in final JSON results

[cjonas9]

What

Following the stepped-pacer redesign (#23):

• Adds a per-step-interval snapshot timeline to the final JSON results. Each snapshot captures target RPS, success/error counts, error rate (%), and latency percentiles (p50, p95, p99, p99.9) for that window only
• Removes the redundant log file mirror (as it's now superseded by the richer JSON)

Visually, the results JSON now looks like:

{
  /*
  Old: cumulative summary data of the entire run
  */
  "start": ...,
  "end": ...,
  "duration_seconds": 123,
  "endpoints": {
    "getLedgers": {
       // summary for the entire run of performance, error breakdowns by type, etc.
      },

    /*
    NEW: data of how the RPC performed at each RPS phase for this endpoint
    */
      "timeline": [
        {
          "target_rps": 7.14, "success": 142, "errors": 0, "error_rate_pct": 0,
          "p50_ms": 165.631, "p95_ms": 537.087, "p99_ms": 900.095, "p99.9_ms": 935.423
        },
        {
          "target_rps": 14.29, "success": 277, "errors": 0, "error_rate_pct": 0,
          "p50_ms": 262.399, "p95_ms": 734.719, "p99_ms": 974.335, "p99.9_ms": 1098.751
        },
        ...,
        {
          "target_rps": 50, "success": 0, "errors": 116, "error_rate_pct": 100,
          "p50_ms": 15007.743, "p95_ms": 15007.743, "p99_ms": 15007.743, "p99.9_ms": 15007.743
        }
      ]
    }
  }
}

Why

The richer JSON gives operators a clear view of how the RPC degraded as load increased. Previously, the results JSON flattened all metrics into a single summary bucket per endpoint without any notion of an RPS-timeline, so there was no way to tell at what RPS the RPC buckled. This gives an operator running a load test a concise/minimal degradation curve (e.g. when errors started, how latency + error rates changed at each step) without requiring them to dig through through logs.

Known limitations

The intervals default to 5s when no ramp-up is provided as there's no natural "RPS phase" notion without a ramp-up stepping through various RPS levels.