Added error code reporting if kinect fails to initialise

Might want to wrap the errors into something more tangible, but it
looked like you wanted to keep win types (DWORD, HRESULT etc) out of the
ofxKinectNui type, so I just returned a string.
Merge pull request #21 from SoylentGraham/Master-Graham

Added error code reporting if kinect fails to initialise