refer to:
http://www.runoob.com/python/python-exceptions.html
http://www.pythondoc.com/pythontutorial3/errors.html
http://www.cnblogs.com/rubylouvre/archive/2011/06/22/2086644.html
python 中的异常处理
原文：http://www.cnblogs.com/qingyuanjushi/p/6422138.html
