ethernet_interface: Emit error when missing MAC

Now that our MAC address is being mocked in our test suite we can
actually error out when the HWADDR info is missing.

Change-Id: I84ac492774ba026194dc90c53f64aa63d9a3b1ca
Signed-off-by: William A. Kennington III <wak@google.com>

ethernet_interface.cpp

@@ -268,15 +268,15 @@ std::string
     {
         log<level::ERR>("socket creation  failed:",
                         entry("ERROR=%s", strerror(errno)));
-        return "";
+        elog<InternalFailure>();
     }
 
     std::strcpy(ifr.ifr_name, interfaceName.c_str());
     if (ioctl(sock, SIOCGIFHWADDR, &ifr) != 0)
     {
         log<level::ERR>("ioctl failed for SIOCGIFHWADDR:",
                         entry("ERROR=%s", strerror(errno)));
-        return "";
+        elog<InternalFailure>();
     }
 
     static_assert(sizeof(ifr.ifr_hwaddr.sa_data) >= sizeof(ether_addr));