examples: wifi-multi-tos prints a message if throughput is zero

otherwise it mysteriously exits with a non-zero error code.

examples/wireless/wifi-multi-tos.cc

@@ -191,7 +191,7 @@ main(int argc, char* argv[])
     }
     else
     {
-        NS_LOG_ERROR("Obtained throughput is 0!");
+        std::cout << "Obtained throughput is 0!" << std::endl;
         exit(1);
     }