Minor changes to .gitignore and error messages

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any):

• Brief description of code changes (suitable for use as a commit message):

Suggested three minor changes. I am sending all in one PR as I didn't see the point in submitting 3 PRs for such minor changes. In practice, I assume that the accepted/revised changes can be done manually directly the the code (and this PR can be closed).

1. Add ".vscode/" to .gitignore. Folder is added by Visual Studio Code, at list for me, and I assume this may be the case for others.
2. Add the return code value of check_authentication() to a debug message, to allow better evaluation of the error.
3. Enhance IESENDMESSAGE and IERECVMESSAGE messages per this suggestion in issue iperf3: error - unable to send control message: Bad file descriptor #1233 (issue may be closed by the way).

[bmah888]

Thanks for the PR! I'm not a huge fan of long error messages, but I understand that inexperienced users might need some more guidance. I'd like it if you could fix a couple of grammatical issues before merging, then we're good.

[bmah888]

Thanks! Merged.