add response body to error if possible

github/actions/client.go

@@ -1056,11 +1056,17 @@ func (c *Client) fetchAccessToken(ctx context.Context, gitHubConfigURL string, c
     defer resp.Body.Close()
 
     if resp.StatusCode != http.StatusCreated {
-		return nil, &GitHubAPIError{
+        rErr := &GitHubAPIError{
             StatusCode: resp.StatusCode,
             RequestID:  resp.Header.Get(HeaderGitHubRequestID),
             Err:        fmt.Errorf("failed to get access token for GitHub App auth: %v", resp.Status),
         }
+        msg, err := io.ReadAll(resp.Body)
+        if err != nil {
+            return nil, rErr
+        }
+        rErr.Err = fmt.Errorf("%w: %s", rErr.Err, msg)
+        return nil, rErr
     }
 
     // Format: https://docs.github.com/en/rest/apps/apps#create-an-installation-access-token-for-an-app