Warns the user that the specified file is not saved. The action argument should be a string that indicates what action is being taken (e.g., ``Quit''). If the user chooses to proceed without saving, the return value is 'continue. If the user chooses to cancel, the return value is 'cancel. If now? is not #f, then the user may also elect to save the file and continue, in which case the return value is 'save.