graph/education: slightly improve error handling and logging

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- Use var for common errors
- Add the addition error message to the Error() output of errorcode.Error
- in PatchEducationSchool() use errorcode.RenderError() to turn the errorcode
  in to the right HTTP Status (instead of return 500 always)

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500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f