openonuAdapterGo - hardening: check ONU OMCI get responses for consistency

In attempting to integrate a new ONU provider, it became clear that we cannot rely on the ONU to actually provide the attributes requested using OMCI get request in every case, even if it marks the response as successful. Therefore, the received attributes should be checked for their existence in the FSMs before being utilized at the final point of use. Appropriate interception strategies should be implemented in case of failures.