Validation fails for failed actions (and possibly tracebacks) with additional extracted fields

[itamarst]

Given an exception extractor, validation of logging in a test started failing:

Traceback (most recent call last):
  File "/home/itamarst/ClusterHQ/flocker/.tox/py27/local/lib/python2.7/site-packages/twisted/trial/_synctest.py", line 1368, in _runCleanups
    f(*args, **kwargs)
  File "/home/itamarst/ClusterHQ/flocker/.tox/py27/local/lib/python2.7/site-packages/eliot/_output.py", line 295, in validate
    serializer.validate(dictionary)
  File "/home/itamarst/ClusterHQ/flocker/.tox/py27/local/lib/python2.7/site-packages/eliot/_validation.py", line 266, in validate
    raise ValidationError(message, "Unexpected field %r" % (key,))
eliot._validation.ValidationError: ({u'action_status': u'failed', u'task_level': [3, 2], u'timestamp': 1449003265.709564, u'task_uuid': u'f6a314ab-2e97-42e6-821c-e30ff2023195', u'action_type': u'api:json_request', u'code': 402, u'exception': u'flocker.restapi._error.BadRequest', u'reason': u"(402, u'additional details here')"}, u"Unexpected field u'code'")