Replace logging.exception with logging.error

[unnonouno]

When I used nosetests command with --stop option, I failed to run my test, and it raises an exception. And, I found that logging.exception must be called from only except clause. So I replaced it with logging.error. https://docs.python.org/2/library/logging.html#logging.Logger.exception

I used python 3.4.5 and nose 1.3.7. You can easily reproduce the same problem with three test case sources which always fail and run nosetests --processes=2 --stop.