nic_router: drop ARP requests when unconfigured

Whenever the nic_router encounters ARP requests on an interface
that does not have a valid IP config it will ignore them. However,
When increasing the verbosity of the component for diagnostic
purposes the resulting 'Bad network protocol' message is misleading.

Issue #4455.

repos/os/src/server/nic_router/interface.cc

@@ -1591,6 +1591,9 @@ void Interface::_handle_eth(Ethernet_frame           &eth,
 			}
 			break;
 		}
+		case Ethernet_frame::Type::ARP: {
+				throw Drop_packet("Ignore ARP request on unconfigured interface");
+		}
 		default:
 
 			throw Bad_network_protocol();