fixed save format, old saves broken sorry

i've improved the save format to make way for future features (such as exporting to web etc) but that means old saves will be broken - sorry to anyone who is using this at the moment. i shouldn't need to make any more breaking changes to the save files now that this is done.

there's also now a loading screen for opening stories, and the error text is bright red so you can report any errors you run into.