Should try to recover when ink flow dies

If the ink flow crashes out due to runtime author error (e.g. trying to `->->` when not in a tunnel), then a story exception is thrown. This is fine, except it shouldn't leave the `Story` in a completely invalid state, because it means that a game can never recover. Instead, it should tidy up after itself, so that, for example, the game can continue to another scene entirely, keep global variable state, and start a new call stack.