Make status code and NGINX error codes on internalError publicly accessible

[arussellf5]

Proposed changes

This will allow users of the plus client to check errors and decide what further action to take. This fix follows the advice of commentators who counsel users of go to assert errors for behavior, not type. https://dave.cheney.net/2014/12/24/inspecting-errors The user would create a simple error interface including Status() and Code() methods and then could use errors.As() to cast the internalError to their own interface type. For example, if the user attempts to delete an upstream server using the client, the user can check the error returned for a http.StatusNotFound code, and if present, can make their application take no further action. If some error status code is returned, then the user can retry the delete. Previously in order to perform this kind of analysis the user would have to resort to string checking on the error: never a good solution.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[nginx-bot]

Hi @arussellf5!

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.