I agree. We should definitely document it as a change, but I don't think there's anything useful you can do in the case of a flush failing anyway, so it seems unlikely that anyone could depend on the return value.

Returning None for success and exception on error sounds like a good change. Technically this is no longer returning zero, as previously documented, so if anyone wants to get *really* pedantic, we're not returning the "failed" result for success ;)