[IMPROVEMENT] Show an error message in case the user already left when kicking

[janssensjelle]

Types of changes

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• Bugfix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality not to work as expected).
• Documentation Update (if none of the other choices applies).

Proposed changes

If you try to kick a user that already left the server, the bot won't respond. This PR adds an error message and a test function for this.

Thanks Rem for the idea.

Checklist

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• I have read and followed the CONTRIBUTING.md
  doc.
• Lint and unit tests pass locally with my changes.
• I have added the necessary documentation (if appropriate).

Additional context

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.68%. Comparing base (d0a28ff) to head (7c02d90).
Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
+ Coverage   53.15%   53.68%   +0.53%     
==========================================
  Files          43       44       +1     
  Lines        2455     2481      +26     
==========================================
+ Hits         1305     1332      +27     
+ Misses       1150     1149       -1     

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[dimoschi]

Looks good!