Fix up get/set_property vfuncs for GDBus

GDBus asserts that a get_property implementation sets an error
if it returns NULL, and that a set_property implementation sets
and error if it returns FALSE. The power plugin was very far
from this requirement, and I have seen crashes of
gnome-settings-daemon due to this, when trying to set screen
brightness in a VM.

https://bugzilla.gnome.org/show_bug.cgi?id=706858