when an HTTP response fails validation..

previously:
* only the 'Location' header was added to the Error object,
  which can be used by an HTTP client to implement redirects
now:
* "error.location" is still present for backward compatability
* "error.headers" contains a map of all headers,
  which provides more detailed information to the library user;
  for example, each redirect may also include 'Set-Cookie' headers