Adapt to psutil 2.0.0 API.

API changed.
https://psutil.googlecode.com/hg/HISTORY

Tested with 0.5.1, 1.2.1, 2.0.0
Fix python trying to use default getattr using psutil 0.5.1.

Don't know why adding "None" as second default helps.
Fix error types.
Merge pull request #336 from herr-biber/adapt-to-psutil2.0.0-api

Adapt to psutil 2.0.0 API.