AWSError incorrectly passes None as a value for 'code' to twisted.web.error.Error

[Jamu Kakar]

A change made in Twisted r28237 makes t.w.e.Error convert values
passed as the 'code' parameter to an int. txAWS passes None by
default which result in an exception being raised:

  ...
  File "/home/jkakar/src/canonical/landscape-source-dependencies/txaws/trunk/txaws/exception.py", line 14, in __init__
    super(AWSError, self).__init__(status, message, response)
  File "/home/jkakar/src/projects/twisted/trunk/twisted/web/error.py", line 44, in __init__
    message = http.responses.get(int(code))