test__logs_failures_from_cancellers spurious - maasserver/utils/tests/test_async.py failure

[ubuntudotcom1]

======================================================================
FAIL: maasserver.utils.tests.test_async.TestDeferredHooks.test__logs_failures_from_cancellers
----------------------------------------------------------------------
_StringException: Traceback (most recent call last):
  File "/tmp/tarmac/branch.Mkejlf/src/maasserver/utils/tests/test_async.py", line 261, in test__logs_failures_from_cancellers
    logger.output)
  File "/tmp/tarmac/branch.Mkejlf/src/maastesting/testcase.py", line 150, in assertDocTestMatches
    expected, self.doctest_flags if flags is None else flags))
  File "/usr/lib/python2.7/dist-packages/testtools/testcase.py", line 406, in assertThat
    raise mismatch_error
MismatchError: Expected:
    Failure when cancelling hook.
    Traceback (most recent call last):
    ...
    maastesting.factory.TestException#...
Got:
    Unhandled error in Deferred:
    ---
    Unhandled Error
    Traceback (most recent call last):
    Failure: twisted.internet.defer.CancelledError:
    <BLANKLINE>
    ---
    Unhandled error in Deferred:
    ---
    Unhandled Error
    Traceback (most recent call last):
    Failure: twisted.internet.defer.CancelledError:
    <BLANKLINE>
    ---
    Failure when cancelling hook.
    Traceback (most recent call last):
      File "/usr/lib/python2.7/dist-packages/twisted/internet/base.py", line 1201, in mainLoop
        self.runUntilCurrent()
      File "/usr/lib/python2.7/dist-packages/twisted/internet/base.py", line 797, in runUntilCurrent
        f(*a, **kw)
      File "/usr/lib/python2.7/dist-packages/crochet/_eventloop.py", line 383, in runs_in_reactor
        d = maybeDeferred(function, *args, **kwargs)
      File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 139, in maybeDeferred
        result = f(*args, **kw)
    --- <exception caught here> ---
      File "/tmp/tarmac/branch.Mkejlf/src/maasserver/utils/async.py", line 213, in _cancel_in_reactor
        hook.cancel()
      File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 459, in cancel
        canceller(self)
      File "/usr/lib/python2.7/dist-packages/mock.py", line 964, in __call__
        return _mock_self._mock_call(*args, **kwargs)
      File "/usr/lib/python2.7/dist-packages/mock.py", line 1019, in _mock_call
        raise effect
    maastesting.factory.TestException#29: