Don't count statistics for failed connections

[jchampio]

Failed connection items often show up in the result JSON with junk data (zero or negative timing values, etc.), which throws off the overall statistics. This patch keeps track of failed connections and excludes them from the stats.

For example, before this patch I would see output like this:

Aggregate results (WebSocket Opening+Closing Handshake)

          Duration:   26821.4 ms
             Total:    200000
           Success:    198639
              Fail:      1361
            Fail %:      0.68
    Handshakes/sec:      7406

     Min: -926893.4 ms   <- negative
      SD:   68743.1 ms   <- bogus
     Avg:   -5067.6 ms   <- negative
  Median:      19.2 ms
  q90   :     219.3 ms
  q95   :     413.3 ms
  q99   :    1636.9 ms
  q99.9 :    5001.8 ms
  q99.99:    5015.8 ms
     Max:    5038.4 ms