add String.valueof() for correct log message

[Farley-Chen]

Fix #21304

logger.error() may not work rightly with logback, because the cause is a exception.
Maybe use String.valueof() to wrap the cause.

---

Please make sure the below checklist is followed for Pull Requests.

• All continuous integration tests are green
• Tests are added where necessary
• The JDL part is updated if necessary
• jhipster-online is updated if necessary
• Documentation is added/updated where necessary
• Coding Rules & Commit Guidelines as per our CONTRIBUTING.md document are followed

When you are still working on the PR, consider converting it to Draft (below reviewers) and adding skip-ci label, you can still see CI build result at your branch.

[DanielFran]

Thanks @Farley-Chen for the contribution