VoEVolumeTest: Adds error return tests.

BUG=367
TESTED=trybots, voe_auto_test
R=henrikg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/19469006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6139 4adac7df-926f-26a2-2b94-8c16560cd09d