New HTTP client wraps connection losts in ResponseFailed exception. fix #373
do no include ResponseFailed if http11 handler is not enabled
Treat responses without content-length or Transfer-Encoding as good responses

There is not a way to determine if responses without Content-Length or Transfer-Encoding are complete, this change treat them as good responses but flags them as "partial".
This is backout change only for this functionality of 3c64a989
test PotentiaDataLoss errors on unbound responses
py26 can not format zero length fields {}
fix crawling tests under twisted pre 11.0.0
remove extra import added by cherry picked changes