Add error message when ELL proxy is not available
Add error message when ELL proxy is not available

At device Forget() method, add error message when ELL proxy object
is not available.

CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source