bluetooth: Better disconnected error messages for GATT operations.

third_party/WebKit/LayoutTests/bluetooth/script-tests/descriptor/io-op-disconnect-called-before.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(
                 characteristic =>
                     characteristic.getDescriptor(user_description.name))
             .then(descriptor => {
               gattServer.disconnect();
               return assert_promise_rejects_with_message(
                   descriptor.CALLS([readValue()|writeValue(val)]),
                   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'));
             });
       });
 }, 'disconnect() called before FUNCTION_NAME. Reject with NetworkError.');