Rework error handling in preparation for py3k changes
Rework error handling in preparation for py3k changes

When the #ifdefs neccesary to support Python 2 and 3 are added,
there will be many more paths through the code. This style of error
handling makes the code easier to read.

keszybz/python-systemd,Awesense/python-systemd,Awesense/python-systemd,davidstrauss/python-systemd,glittershark/python-systemd,systemd/python-systemd,keszybz/python-systemd,again4you/python-systemd,systemd/python-systemd,glittershark/python-systemd,davidstrauss/python-systemd,again4you/python-systemd,Awesense/python-systemd