WT_SESSION.strerror

@michaelcahill, I've kind of decided I like this change – if you still want it, maybe Don could review & fix Java as necessary?

One important note – I changed the extension API to match the new WT_SESSION method. I'm not sure that's a good idea or not (regardless, I'm thinking the session API isn't widely enough used that we need to worry about backward compatibility).

Reference WT-1516.