Prompts for password.
Set the various properties on the prompt before calling this method to explain which password should be entered.
This method will block until the a response is returned from the prompter.
A password will be returned if the user enters a password successfully. The returned password is valid until the next time a method is called to display another prompt.
null will be returned if the user cancels or if an error occurs. Check the throws argument to tell the difference.
optional cancellation object
the password owned by the prompt, or null