pageserver: add GcError type

[jcsp]

Problem

• Because GC exposes all errors as an anyhow::Error, we have intermittent issues with spurious log errors during shutdown, e.g. in this failure of a performance test https://neon-github-public-dev.s3.amazonaws.com/reports/main/9300804302/index.html#suites/07874de07c4a1c9effe0d92da7755ebf/214a2154f6f0217a/

Gc failed 1 times, retrying in 2s: shutting down

GC really doesn't do a lot of complicated IO: it doesn't benefit from the backtrace capabilities of anyhow::Error, and can be expressed more robustly as an enum.

Summary of changes

• Add GcError type and use it instead of anyhow::Error in GC functions
• In gc_iteration_internal, return GcError::Cancelled on shutdown rather than Ok(()) (we only used Ok before because we didn't have a clear cancellation error variant to use).
• In gc_iteration_internal, skip past timelines that are shutting down, to avoid having to go through another GC iteration if we happen to see a deleting timeline during a GC run.
• In refresh_gc_info_internal, avoid an error case where a timeline might not be found after being looked up, by carrying an Arc instead of a TimelineId between the first loop and second loop in the function.
• In HTTP request handler, handle Cancelled variants as 503 instead of turning all GC errors into 500s.

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]

3156 tests run: 3017 passed, 0 failed, 139 skipped (full report)

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Code coverage* (full report)

• functions: 31.4% (6505 of 20711 functions)
• lines: 48.3% (50226 of 103898 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
950e5d2 at 2024-05-31T17:05:26.139Z :recycle:}