syncer: remove improper ignored error (#1585)

[ti-srebot]

cherry-pick #1585 to release-2.0
You can switch your code base to this Pull Request by using git-extras:

# In dm repo:
git pr https://github.com/pingcap/dm/pull/1599

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

git push git@github.com:ti-srebot/dm.git pr/1599:release-2.0-89e7399f2500

---

What problem does this PR solve?

close #1392

What is changed and how it works?

3 errors should not be ignored: ErrDatabaseNotExists, ErrTableNotExists, ErrColumnNotExists. These errors are not caused by re-execute DDL.

Check List

Tests

• Integration test

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note

[lance6716]

/run-all-tests

[lance6716]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• lance6716

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[lance6716]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 6dce61d