handle get sn error gracefully.

server.js

@@ -342,14 +342,14 @@ function requestHandler(req, res) {
     } else {
         exec ('/usr/bin/unit_serial_number', function (error, stdout, stderr) {
             var sn = stdout.trim();
-            if(!sn) {
+            if(error || !sn ) {
+                console.log("error getting serial number!", error);
                 sn = "unknown"
             }
 
             var res_str = getStateBasedIndexPage();
             var host_url = sn + ".local";
             var host_port = NODE_RED_PORT.toString().trim();
-            console.log("host_url=%s, port=%s", host_url, host_port);
             
             res_str = res_str.replace(/params_hostname/g, host_url);
             res_str = res_str.replace(/params_node_red_http_port/g, host_port);