GetWaitPageMessage returns a specific static message for display in the wait page. This implementation returns the default wait page message (which can be set in the constructor), optionally combined with the current ProgressMessage, if the _progressMessage flag has been set to true in the constructor.
This function returns the specific waitPage for this ITaskWaiter, and is used in case a different WaitPage is required for a particular task waiter, instead of the default one that the wait page mechanism is initially set up with. This implementation returns null, which indicates that the default wait page should be used. Override this function to provide a specific WaitPage for the task.