Fix bogus 'invalid response' message on URL rewriter interface

Fix bogus 'invalid response' message on URL rewriter interface

The empty-line response from rewriter and redirector should be converted
to ERR reply code in the new API. It was being left as Unknown.

While this reply used to only be valid on URL helper interface, and it
woudl be more appropriate to map other helpers to BH the ERR response
seems to be safe for use on any of the helper interfaces for an empty
line response. At worst it will prevent the lookup being re-tried on
other possibly better working helper instance.