small fix for better error handling with a nonexistant backend/device
small fix for better error handling with a nonexistant backend/device

sreichholf/python-coherence,opendreambox/python-coherence,furbrain/Coherence,sreichholf/python-coherence,unintended/Cohen,ismaelgaudioso/Coherence,coherence-project/Coherence,coherence-project/Coherence,unintended/Cohen,opendreambox/python-coherence,furbrain/Coherence,ismaelgaudioso/Coherence