bluetooth: Better disconnected error messages for GATT operations.

third_party/WebKit/LayoutTests/bluetooth/script-tests/characteristic/gatt-op-device-reconnects-during-success.js

 promise_test(() => {
   let val = new Uint8Array([1]);
   return setBluetoothFakeAdapter('DisconnectingDuringSuccessGATTOperationAdapter')
     .then(() => requestDeviceWithKeyDown({
       filters: [{services: ['health_thermometer']}]}))
     .then(device => device.gatt.connect())
     .then(gatt => gatt.getPrimaryService('health_thermometer'))
     .then(service => service.getCharacteristic('measurement_interval'))
     .then(characteristic => {
       let disconnected = eventPromise(characteristic.service.device, 'gattserverdisconnected');
       let promise = assert_promise_rejects_with_message(
         characteristic.CALLS([
           readValue()|
           writeValue(val)|
           startNotifications()]),
-        new DOMException('GATT Server disconnected while performing a GATT operation.',
+        new DOMException('GATT Server is disconnected. Cannot perform GATT operations. ' +
+                         '(Re)connect first with `device.gatt.connect`.',
                          'NetworkError'));
       return disconnected.then(() => characteristic.service.device.gatt.connect())
                          .then(() => promise);
     });
 }, 'Device reconnects during a FUNCTION_NAME call that succeeds. Reject ' +
    'with NetworkError.');