syncer: fix ignore canceled error (#401)

[sre-bot]

cherry-pick #401 to release-1.0

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What problem does this PR solve?

in #355, ignore canceled errors for executing SQLs is introduced.

but miss to ignore the canceled errors for executing DMLs.

What is changed and how it works?

check !utils.IsContextCanceledError(err) before s.runFatalChan <- unit.NewProcessError(errType, err) for executing DMLs as we did for DDL.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  • hack the code for executing DMLs and return an error in one goroutine
  • obverse the log and result of query-status for context canceled error

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note (with loader/syncer: filter context cancel error while executing sqls #355)

[csuzhangxc]

/run-all-tests tidb=release-3.0

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-1.0@3dd5fa5). Click here to learn what that means.
The diff coverage is 0%.

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[csuzhangxc]

@WangXiangUSTC @amyangfei PTAL

[amyangfei]

LGTM

[WangXiangUSTC]

LGTM