introduce region error handler unit test

[iosmanthus]

Signed-off-by: iosmanthus [email protected]<!-- Thank you for contributing to TiKV Java Client!

PR Title Format: "[close/to/fix #issue_number] summary" -->

What problem does this PR solve?

Issue Number: close #issue_number

Problem Description: TBD

What is changed and how does it work?

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has methods of interface change
• Has persistent data change
• No code

Check List for Tests

This PR has been tested by at least one of the following methods:

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Possible performance regression, WHY: TBD
• Increased code complexity, WHY: TBD
• Breaking backward compatibility, WHY: TBD
• NO side effects

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note
• NO related changes

[codecov]

Codecov Report

All modified lines are covered by tests ✅

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