pageserver: improved synthetic size & find_gc_cutoff error handling

[jcsp]

Problem

This PR refactors some error handling to avoid log spam on tenant/timeline shutdown.

• "ignoring failure to find gc cutoffs: timeline shutting down." logs (ignoring failure to find gc cutoffs: timeline shutting down should be info! level #8012)
• "synthetic_size_worker: failed to calculate synthetic size for tenant ...: Failed to refresh gc_info before gathering inputs: tenant shutting down", for example here: https://neon-github-public-dev.s3.amazonaws.com/reports/pr-8049/9502988669/index.html#suites/3fc871d9ee8127d8501d607e03205abb/1a074a66548bbcea

Closes: #8012

Summary of changes

• Refactor: Add a PageReconstructError variant to GcError: this is the only kind of error that find_gc_cutoffs can emit.
• Functional change: only ignore shutdown PageReconstructError variant: for other variants, treat it as a real error
• Refactor: add a structured CalculateSyntheticSizeError type and use it instead of anyhow::Error in synthetic size calculations
• Functional change: while iterating through timelines gathering logical sizes, only drop out if the whole tenant is cancelled: individual timeline cancellations indicate deletion in progress and we can just ignore those.

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]

3216 tests run: 3074 passed, 0 failed, 142 skipped (full report)

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Code coverage* (full report)

• functions: 31.5% (6636 of 21065 functions)
• lines: 48.6% (51637 of 106275 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
12d5128 at 2024-06-13T19:22:14.109Z :recycle:}