spapi: fix DeprecationWarning: BaseException.message
spapi: fix DeprecationWarning: BaseException.message

on python 2.6 (centos 6), if there is None in come attributes, this cause
DeprecationWarning: BaseException.message has been deprecated as of Python 2.6
This warinng stops latthreshold.py if a client is missing. On python 2.7 there
is not such warning.

Change-Id: I9f669aec5dacde48ced45550c3c58ea5be0e733b

storpool/python-storpool