Thanx @cevich and @yumingfei 
I am always considering about this issue from Chris pointed it out.
And I read the online doc for code_style of autotest.
https://github.com/autotest/autotest/blob/master/CODING_STYLE

After considering again and again, I think I was really creating too many error class. 
I believe we can use error type according this sequence:
(1). what Python listed on http://docs.python.org/2/library/exceptions.html
(2). what contained in autotest/client/shared/error.py
(3). After all, If we think it's really really necessary, we can externed error type in error.py.
      Even we can add a error type in error.py if the new error type is widely used.

So, I will do some cleanup in my code. Thanx again.