ETSI20: Improved error reporting between border nodes

The border nodes use the kmsintercom protocol to communicate ETSI20 related information between the ingress and the egress border node in the KeyDeliveryRequest and the corresponding KeyDeliveryResponse.

However, right now any failure, i.e., whatever the failure is, is solely signaled from the egress node with the e20FwdOk set to false.

A better reporting, e.g., adding the error text to the KeyDeliveryResponse would be benefical, as the ingress border node could react differently.