bluetooth: Better disconnected error messages for GATT operations.

third_party/WebKit/LayoutTests/bluetooth/script-tests/characteristic/gatt-op-device-disconnects-before.js

 'use strict';
 promise_test(() => {
   let val = new Uint8Array([1]);
   return setBluetoothFakeAdapter('DisconnectingHealthThermometerAdapter')
     .then(() => requestDeviceWithKeyDown({
       filters: [{services: ['health_thermometer']}],
       optionalServices: [request_disconnection_service_uuid]
     }))
     .then(device => device.gatt.connect())
     .then(gattServer => {
       let measurement_interval;
       return gattServer.getPrimaryService('health_thermometer')
         .then(ht=> ht.getCharacteristic('measurement_interval'))
         .then(mi => measurement_interval = mi)
         .then(() => get_request_disconnection(gattServer))
         .then(requestDisconnection => requestDisconnection())
         .then(() => 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 is disconnected. Cannot perform GATT operations.',
+            'GATT Server is disconnected. Cannot perform GATT operations. ' +
+            '(Re)connect first with `device.gatt.connect`.',
             'NetworkError')));
     });
 }, 'Device disconnects before FUNCTION_NAME. Reject with NetworkError.');