fix: Not write UIDL sync message when RPC handling throws

[mshabarov]

Description

Liferay container is so strict regards ServletResponse::setContentLength comparing to normal servlet containers and Apache Pluto portal, thus the PortalUidlRequestHandler works incorrectly in case of exception in the server-side listener: it writes the error meta info JSON into response, then appends sync UIDL JSON message, then calls setContentLength with the length of the sync UIDL message. Which in turn leads to an unpredictable JSON sent to client.

When this change is applied, UidlRequestHandler doesn't write sync UIDL message to the response once the RPC handler execution throws, but instead sends only an error message to the client.

No IT tests added, because this is covered by LiferayErrorHandlingIT being added afterwards within Liferay tests PR.

Fixes #213

Type of change

• [x] Bugfix
• [ ] Feature

Checklist

• [x] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• [x] I have added a description following the guideline.
• [x] The issue is created in the corresponding repository and I have referenced it.
• [ ] I have added tests to ensure my change is effective and works as intended.
• [ ] New and existing tests are passing locally with my change.
• [x] I have performed self-review and corrected misspellings.

Additional for Feature type of change

• [ ] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

[vaadin-bot]

SonarQube analysis reported 5 issues

1. DefaultPortletErrorHandler.java#L29: Invoke method(s) only conditionally.
2. PortletUidlRequestHandler.java#L162: Add the "@Override" annotation above this method signature
3. PortletUidlRequestHandler.java#L166: Add the "@Override" annotation above this method signature
4. VaadinPortletService.java#L77: Rename this field "DEFAULT_HANDLER" to match the regular expression '^[a-z][a-zA-Z0-9]*$'.
5. VaadinPortletService.java#L256: Complete the task associated to this TODO comment.