bluetooth: Better disconnected error messages for GATT operations.

third_party/WebKit/LayoutTests/bluetooth/script-tests/characteristic/gatt-op-disconnect-called-during-success.js

 'use strict';
 promise_test(() => {
   let val = new Uint8Array([1]);
   return setBluetoothFakeAdapter('DisconnectingHealthThermometerAdapter')
     .then(() => requestDeviceWithKeyDown({
       filters: [{services: ['health_thermometer']}]}))
     .then(device => device.gatt.connect())
     .then(gattServer => {
       return gattServer
         .getPrimaryService('health_thermometer')
         .then(service => service.getCharacteristic('measurement_interval'))
         .then(measurement_interval => {
           let promise = assert_promise_rejects_with_message(
             measurement_interval.CALLS([
               getDescriptor(user_description.name)|
               getDescriptors(user_description.name)[UUID]|
               getDescriptors()|
               readValue()|
               writeValue(val)|
               startNotifications()|
               stopNotifications()]),
             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'));
           gattServer.disconnect();
           return promise;
         });
     });
 }, 'disconnect() called during a FUNCTION_NAME call that succeeds. ' +
    'Reject with NetworkError.');