fix: prevent panic on partial append failures

During partial append failures, the error field was incorrectly accessed
from a nil err variable instead of the result struct, causing a nil
pointer dereference panic.

Change err.Error() to res[idx].Error to properly handle error messages
in partial failure cases.

pkg/varlog/operations.go

@@ -74,7 +74,7 @@ RETRY:
 
 		for idx := 0; idx < len(res); idx++ {
 			if len(res[idx].Error) > 0 {
-				if strings.Contains(err.Error(), "sealed") {
+				if strings.Contains(res[idx].Error, "sealed") {
 					result.Err = fmt.Errorf("append: %s: %w", res[idx].Error, verrors.ErrSealed)
 				} else {
 					result.Err = fmt.Errorf("append: %s", res[idx].Error)