pkg/lightning: fix a bug that importer backend may ignore write rows error (#835)

[ti-srebot]

cherry-pick #835 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In br repo:
git pr https://github.com/pingcap/br/pull/848

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/br.git pr/848:release-4.0-a0b0388ee62c

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What problem does this PR solve?

• Fix a bug that importer backend may ignore write rows error when open engine returns file exists error
• Adjust several checkpoint checks to avoid reopen index engine after the engine is closed.

What is changed and how it works?

Check List

Tests

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

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

Release Note

• Fix the bug that importer may ignore write rows error if open engine returns file exists error

[overvenus]

LGTM

[kennytm]

/lgtm