Log:
https://chrome-swarming.appspot.com/task?id=3ec720e6e6984710&refresh=10&show_raw=1

Traceback (most recent call last):
  RunBenchmark at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/story_runner.py:366
    expectations=expectations, max_num_values=benchmark.MAX_NUM_VALUES)
  Run at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/story_runner.py:214
    test, finder_options.Copy(), story_set)
  traced_function at /b/swarming/w/ir/third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py:52
    return func(*args, **kwargs)
  __init__ at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/page/shared_page_state.py:86
    self.platform.network_controller.Open(wpr_mode)
  traced_function at /b/swarming/w/ir/third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py:52
    return func(*args, **kwargs)
  Open at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/core/network_controller.py:28
    self._network_controller_backend.Open(wpr_mode)
  Open at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/platform/network_controller_backend.py:70
    local_port = self._StartTsProxyServer()
  _StartTsProxyServer at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/platform/network_controller_backend.py:206
    self._ts_proxy_server.StartServer()
  StartServer at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/util/ts_proxy_server.py:98
    'Error starting tsproxy: %s' % err)
RuntimeError: Error starting tsproxy: None

Locals:
  cmd_line : ['/b/swarming/w/ir/.swarming_module_cache/vpython/09eff0/bin/python', '/b/swarming/w/ir/third_party/catapult/telemetry/third_party/tsproxy/tsproxy.py', '--port=0', '--desthost=127.0.0.1']
  err      : None
  fd       : 16
  fl       : 0
  timeout  : 10

Suspect this has to do with my change recently to changes the way we read output from tsproxy