Add uri to log when 'error getting stats' occured for nginx

[AlexKorsakov]

Add uri to error message which occurs when exporter couldn't get success status code from one of the addresses for scrape.

1. GetApiEndpoint() created because apiEndpoint is unexported.
2. Added attribute uri to log
3. Pass err object instead of err.Error() to save stacktrace. Or is there any cause not to change it?
4. I haven't made the same changes for nginx_plus.go yet, cause it requires to change plusclient but I don't have any license to run and test it after changes. I can make it without testing, only run lint and build.

[nginx-bot]

Hi @AlexKorsakov! Welcome to the project! 🎉

Thanks for opening this pull request!
Be sure to check out our Contributing Guidelines while you wait for someone on the team to review this.

[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[AlexKorsakov]

I have hereby read the F5 CLA and agree to its terms

[ciarams87]

lgtm once the lint issue is resolved, thanks!

[AlexKorsakov]

lgtm once the lint issue is resolved, thanks!

Finished. Check again, please, @ciarams87 .