[py] Add detailed error message if KeyError thrown in get_window_size and get_window_position (#15503)#15504

aikfiend wants to merge 1 commit intoSeleniumHQ:trunkSeleniumHQ/selenium:trunkfrom

aikfiend:add-window-size-and-position-error-messagesaikfiend/selenium:add-window-size-and-position-error-messagesCopy head branch name to clipboard