Proxy should not log tracebacks on EPIPE while sending to object-servers

This is an entirely expected category of error, and the existing traceback doesn't provide anything useful:

Jul 21 20:50:47 saio proxy-server: ERROR with Object server 127.0.0.4:6040/sdb4 re: Trying to write to /AUTH_test/c/o:
Traceback (most recent call last):
  File "/vagrant/swift/swift/proxy/controllers/obj.py", line 1670, in _send_chunk
    self.conn.send(to_send)
  File "/usr/lib/python2.7/httplib.py", line 870, in send
    self.sock.sendall(data)
  File "/usr/local/lib/python2.7/dist-packages/eventlet/greenio/base.py", line 403, in sendall
    tail = self.send(data, flags)
  File "/usr/local/lib/python2.7/dist-packages/eventlet/greenio/base.py", line 397, in send
    return self._send_loop(self.fd.send, data, flags)
  File "/usr/local/lib/python2.7/dist-packages/eventlet/greenio/base.py", line 384, in _send_loop
    return send_method(data, *args)
error: [Errno 32] Broken pipe (txn: txa363f55c9629439282a10-005f175525) (client_ip: 127.0.0.1)

Happens on both replicated and EC