Ranker throws exception if program bot stops early

[Procrat]

This is not fatal for the ranker (as it always restarts after 5 minutes), but it is fatal for the match.

Traceback (most recent call last):
  File "/home/battlebots/production/battlebots/ranker/ranker.py", line 127, in battle_loop
    battle(loop)
  File "/home/battlebots/production/battlebots/ranker/ranker.py", line 91, in battle
    loop.run_until_complete(game.play_async())
  File "/usr/lib/python3.4/asyncio/base_events.py", line 276, in run_until_complete
    return future.result()
  File "/usr/lib/python3.4/asyncio/futures.py", line 277, in result
    raise self._exception
  File "/usr/lib/python3.4/asyncio/tasks.py", line 235, in _step
    result = coro.send(value)
  File "/home/battlebots/production/battlebots/arbiter/arbiter.py", line 476, in play_async
    yield from player.stop_process()
  File "/home/battlebots/production/battlebots/arbiter/arbiter.py", line 383, in stop_process
    self.process._transport.close()
  File "/usr/lib/python3.4/asyncio/base_subprocess.py", line 76, in close
    self.terminate()
  File "/usr/lib/python3.4/asyncio/base_subprocess.py", line 94, in terminate
    self._proc.terminate()
  File "/usr/lib/python3.4/subprocess.py", line 1686, in terminate
    self.send_signal(signal.SIGTERM)
  File "/usr/lib/python3.4/subprocess.py", line 1681, in send_signal
    os.kill(self.pid, sig)
ProcessLookupError: [Errno 3] No such process