fix(pageserver): passthrough partition cancel error

[skyzh]

Problem

close #9142

Summary of changes

passthrough CollectKeyspaceError::Cancelled to CompactionError::ShuttingDown

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

5039 tests run: 4875 passed, 0 failed, 164 skipped (full report)

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Flaky tests (4)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_subscriber_restart: release-x86-64

Postgres 16

• test_slots_and_branching: release-x86-64

Postgres 14

• test_multixid_wraparound_import: release-x86-64

Code coverage* (full report)

• functions: 32.0% (7495 of 23396 functions)
• lines: 50.0% (60485 of 120856 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
b0f5962 at 2024-09-25T18:40:12.297Z :recycle:}