Fix nits in Web Bluetooth sample

[beaufortfrancois]

function readBatteryLevel() {
         services: ['battery_service']
       }]
     })
-    .then(function(device) {
+    .then(function (device) {
       return device.gatt.connect();
     })
-    .then(function(server) {
+    .then(function (server) {
       return server.getPrimaryService('battery_service');
     })
-    .then(function(service) {
-      if (!service) {
-        throw 'Battery Service not found';
-      }
+    .then(function (service) {
       return service.getCharacteristic('battery_level');
     })
     .then(function (characteristic) {
@@ -29,7 +26,7 @@ function readBatteryLevel() {
     .then(function (value) {
       $target.innerHTML = 'Battery percentage is <b>' + value.getUint8(0) + '</b>.';
     })
-    .catch(function(error) {
+    .catch(function (error) {
       $target.innerText = error;
     });
 }

[NOtherDev]

👍 Done.