Readline read_history and write_history addition

[Yitzchak Gale]

> > Those are asynchronous phenomena.

> I'm not sure what you mean by that?
> It's a synchronous exception.

Sorry. I meant external. In the following sense:

The semantics of the program explicitly involve
it in a larger external system, in a way that it
might make sense to allow exceptions to propagate
out to the external system.

With readline history, it is clear that we must handle
all conditions internally, so this condition is not met.
Since E1-3 are also not met, we should not throw
IO exceptions.

-Yitz