bluetooth: Better disconnected error messages for GATT operations.

third_party/WebKit/LayoutTests/bluetooth/script-tests/descriptor/io-op-device-disconnects-during-success.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 => {
             return gattServer.getPrimaryService('health_thermometer')
                 .then(ht => ht.getCharacteristic('measurement_interval'))
                 .then(
                     characteristic =>
                         characteristic.getDescriptor(user_description.name))
                 .then(d => user_description = d)
                 .then(() => get_request_disconnection(gattServer))
                 .then(requestDisconnection => {
                   requestDisconnection();
                   return assert_promise_rejects_with_message(
                       user_description.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'));
                 });
           });
     },
     'Device disconnects during a FUNCTION_NAME call that succeeds. ' +
         'Reject with NetworkError.');