The page at
http://docs.python.org/dev/library/unittest.html#module-unittest says:

-------------
TestResult.addError(test, err)

    Called when the test case test raises an unexpected exception err is
a tuple of the form returned by sys.exc_info(): (type, value, traceback).

    The default implementation appends (test, err) to the instance's
errors attribute.
--------------

Starting from 2.2, a formatted traceback is added to the "error"
attribute instead of the actual sys.exc_info(). The same error is
present for addFailure() as well.