bluetooth: Better disconnected error messages for GATT operations.

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

 'use strict';
 promise_test(() => {
   let val = new Uint8Array([1]);
   return setBluetoothFakeAdapter('FailingGATTOperationsAdapter')
     .then(() => requestDeviceWithKeyDown({
       filters: [{services: [errorUUID(0xA0)]}],
       optionalServices: [request_disconnection_service_uuid]
     }))
     .then(device => device.gatt.connect())
     .then(gattServer => {
       let error_characteristic;
       return gattServer
         .getPrimaryService(errorUUID(0xA0))
         .then(es => es.getCharacteristic(errorUUID(0xA1)))
         .then(ec => error_characteristic = ec)
         .then(() => get_request_disconnection(gattServer))
         .then(requestDisconnection => {
           requestDisconnection();
           return assert_promise_rejects_with_message(
             error_characteristic.CALLS([
               getDescriptor(user_description.name)|
               getDescriptors(user_description.name)[UUID]|
               getDescriptors()|
               readValue()|
               writeValue(val)|
               startNotifications()]),
             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 fails. ' +
    'Reject with NetworkError.');