bluetooth: Better disconnected error messages for GATT operations.

third_party/WebKit/LayoutTests/bluetooth/script-tests/descriptor/io-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 =>
                   characteristic.getDescriptor(user_description.name))
           .then(descriptor => {
             let disconnected = eventPromise(
                 descriptor.characteristic.service.device,
                 'gattserverdisconnected');
             let promise = assert_promise_rejects_with_message(
                 descriptor.CALLS([readValue()|writeValue(val)]),
                 new DOMException(
-                    'GATT Server disconnected while performing a GATT operation.',
+                  'GATT Server is disconnected. Cannot perform GATT operations. ' +
+                    '(Re)connect first with `device.gatt.connect`.',
                     'NetworkError'));
             return disconnected
                 .then(
                     () =>
                         descriptor.characteristic.service.device.gatt.connect())
                 .then(() => promise);
           });
     },
     'Device reconnects during a FUNCTION_NAME call that succeeds. Reject ' +
         'with NetworkError.');