Correct import error of pidfile in daemon, python can't find file.

I want use daemoinc in my project, but python crash with error : "'module' object has no attribute PiDFileError".

This error happens becausse i have install  https://github.com/bmhatfield/python-pidfile module, and python import first this module.

This patch correct the conffusion, but a think is a good idea to change the name of pidfile.py in pidfileerror.py.

Best regard