Failure to set direction results in an unhelpful failure from a seemingly infallible method

https://github.com/harfbuzz/harfrust/blob/d47a254fbf173b1931211df282c14e1c45dea1c1/src/hb/ot_shape_plan.rs#L50 failing is spectacularly unhelpful. An infallible looking API can fail, and when it does what prints offers no guidance wrt what to do. Ideally we'd avoid being in a bad state but if we need to allow for it then lets use Result?