pkg/schedule, grpc_service : Add error code if scatter_region grpc is failed (#6953)

[ti-chi-bot]

This is an automated cherry-pick of #6953

What problem does this PR solve?

Scatter_region grpc doesn't fail and return success. It prevents clients to correctly retry the requests and eventually regions are skewed to few nodes.

Issue Number: Close #6950 6950

What is changed and how does it work?

Return the error code if it fails.

Check List

Tests

• manual test
• Steps

1. Set the store limit to 5
2. Run the pre split sql command with 32 regions.
3. Verify that regions are scattered by using get_region.

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci/build	0439df5	link	true	/test build

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[nolouch]

ref #7073

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

1. It must be approved by the approvers firstly.
2. AFTER it has been approved by approvers, please wait for the cherry-pick merging approval from triage owners.

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