storage controller: log hygiene & better error type

[jcsp]

These are testability/logging improvements spun off from #7475

• Don't log warnings for shutdown errors in compute hook
• Revise logging around heartbeats and reconcile_all so that we aren't emitting such a large volume of INFO messages under normal quite conditions.
• Clean up the last_error of TenantShard to hold a ReconcileError instead of a String, and use that properly typed error to suppress reconciler cancel errors during reconcile_all_now. This is important for tests that iteratively call that, as otherwise they would get 500 errors when some reconciler in flight was cancelled (perhaps due to a state change on the tenant shard starting a new reconciler).

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]

2790 tests run: 2670 passed, 0 failed, 120 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_compute_pageserver_connection_stress: debug
• test_pageserver_metrics_removed_after_detach: debug

Postgres 15

• test_partial_evict_tenant[relative_equal]: release

Postgres 14

• test_timeline_size_quota_on_startup: release

Code coverage* (full report)

• functions: 28.1% (6484 of 23066 functions)
• lines: 46.9% (46073 of 98228 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
caad0a9 at 2024-04-26T08:27:44.710Z :recycle:}

[VladLazar]

Looks good, but it made a test unhappy