[MERGE][#263527] Do not traceback on unexpected error responses from a smart server.

[Andrew Bennetts]

John Arbash Meinel wrote:
[...]
> 
> BB:tweak
> 
> Any reason not to use: UnknownErrorFromSmartServer?
> 
> It seems a little bit easier than "Untranslateable".
> 
> I'm fine either way, and I agree that it is the correct route to go. I
> thought the whole case was uncaught, but I was misunderstanding the logic.

I'm equally happy with Unknown rather than Untranslateable.  I'll change that
and send it to PQM.

Thanks for the review!

-Andrew.