PassthroughErrorHandler shouldn't pass an error if response is non-nil

The README for this package reads:

> [...] if an error is returned by the client (connection errors, etc.), or if a 500-range response code is received (except 501), then a retry is invoked after a wait period. Otherwise, the response is returned and left to the caller to interpret.

However, as it stands, `PassthroughErrorHandler` doesn't seem to behave in a way that would accomplish this behaviour out of the box under normal circumstances. This is because it always passes to the caller of `RoundTrip` the final `http.Response` and the error from the call to `CheckRetry`. A default `http.Client` [will actually throw away the response](https://cs.opensource.google/go/go/+/refs/tags/go1.17.8:src/net/http/client.go;l=255;drc=refs%2Ftags%2Fgo1.17.8) if an error is returned. So in the case where, for example, we exhaust all `RetryMax` attempts due to consecutive 5xx errors, the caller will never see the final response body.

`PassthroughErrorHandler` could be much more useful if it first checks for a non-`nil` response:

```go
func PassthroughErrorHandler(resp *http.Response, err error, _ int) (*http.Response, error) {
if resp != nil {
return resp, nil
}
return resp, err
}
```

This throws away the helpful `CheckRetry` error on the last request, but gives the caller the expected ability to distinguish unexpected HTTP status codes from connection errors. Or maybe I'm misinterpreting what purpose `PassthroughErrorHandler` has?