Use correct error in debug log for request failure

[keitwb]

It was using plain err twice and never logging the retryErr.

[twmb]

Nice find, thank you. Were you looking into a problem to find this, or just browsing the code?

[twmb]

This will be out in the next release, either a patch release if I fix only #190 and then release, or a feature release if I address everything else right after it. #190 is tricky.

[keitwb]

Nice find, thank you. Were you looking into a problem to find this, or just browsing the code?

I was trying to debug issues with my kafka cluster setup and so I turned on debug logs for franz and noticed that it was showing retries but the error fields in the logs were both nil so I couldn't figure out why it was retrying until I noticed the duplication of err.