[SPARK-33198][CORE] getMigrationBlocks should not fail at missing files

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to fix getMigrationBlocks error handling and to add test coverage.

1. getMigrationBlocks should not fail at indexFile only case.
2. assert causes java.lang.AssertionError which is not an Exception.

Why are the changes needed?

To handle the exception correctly.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Pass the CI with the newly added test case.

[holdenk]

LGTM pending CI

[dongjoon-hyun]

Thank you for review and approval, @holdenk !

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/34662/

[SparkQA]

Kubernetes integration test status failure
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[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/34664/

[SparkQA]

Kubernetes integration test status success
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[dongjoon-hyun]

GitHub Action and K8s IT passed. Merged to master.

[SparkQA]

Test build #130053 has finished for PR 30110 at commit 824d868.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #130054 has finished for PR 30110 at commit 69bf6c0.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #130055 has finished for PR 30110 at commit 73864b0.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.