Example objects has wrong error code

[danielkilbo]

The example objects returns LWM2M_ERROR if a resource is not found.
Then lwm2m-engine returns internal server error on the request.
In case of an object har an optional resource the Leshan server will not read out any data at all due to the internal server error responce.
All objects should return LWM2M_STATUS_NOT_FOUND if the resource is not found.