feat: add GetCodeownersErrors to RepositoriesService (#2405)

[cbrgm]

This PR adds a new function GetCodeownersErrors to the RepositoriesService struct.
It adds support for the endpoint https://docs.github.com/en/rest/repos/repos#list-codeowners-errors as described in #2405

[codecov]

Codecov Report

Merging #2408 (0d5cc3a) into master (fd22ee9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2408   +/-   ##
=======================================
  Coverage   98.06%   98.06%           
=======================================
  Files         119      120    +1     
  Lines       10546    10558   +12     
=======================================
+ Hits        10342    10354   +12     
  Misses        140      140           
  Partials       64       64           

Impacted Files	Coverage Δ
github/repos_codeowners.go	100.00% <100.00%> (ø)

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[gmlewis]

This is looking great, @cbrgm - thank you!
Just a few minor tweaks, please, before we have a second reviewer approval.

[gmlewis]

Thank you, @cbrgm !
LGTM.

Awaiting LGTM+Approval from any other contributor to this repo before merging.

[gmlewis]

Thank you, @spikeb4 !
Merging.