Cancel a request if an error response is seen early

[GoogleCodeExporter]

While talking with breser about error handling on the server for PUT requests, 
I thought of something cool we could do:

Consider a 1 gigabyte PUT request, and *while* we are transferring that data, a 
500 response comes back. If we're using a chunked request, then we could simply 
terminate the request.

(if the request is using C-L, then we'd need to finish the request, or close 
the connection mid-request)

Not exactly sure how to implement this within serf's model. Somehow, the 
chunk_bucket needs to learn of the error and terminate the request stream.

Original issue reported on code.google.com by gstein on 14 Nov 2012 at 6:34