Add response body to error when fetching access token

[Nashluffy]

We spent some time trying to debug why we were getting a 403 from GitHub when the credentials worked fine elsewhere, would be super useful if the body was present in the error.

[nikola-jokic]

If the response status is not created, and if reading the body fails, we would lose the error type GitHubAPIError which contains the RequestID. This request ID is important because we can troubleshoot the issues related to the request on the back end.

Maybe something along the lines of:

errMsg := fmt.Sprintf("failed to get access token for GitHub App auth (%v)", resp.Status)
if body, err := io.ReadAll(resp.Body); err == nil {
  errMsg = fmt.Sprintf("%s: %s", errMsg, string(body))
}

return nil, &GitHubAPIError{
  StatusCode: resp.StatusCode,
  RequestID:  resp.Header.Get(HeaderGitHubRequestID),
  Err:        errors.New(errMsg),
}