* `Response#error`, which, coupled with `ErrorResponse#error`, allows
for `if response.error` kind of conditional;
* `Response#raise_for_status` now returns the response when no error is
raise (for method chaining);
Closes#153
replacing them with SimpleDelegator where it made sense, and removing
them altogether whenever possible. ErrorResponse doesn't have a special
handling for Response methods anymore, but it's highly debatable if it
should.