pageserver: avoid spurious "bad state" logs/errors during shutdown

[jcsp]

Problem

• Initial size calculations tend to fail with Bad state (not active)

Closes: #7911

Summary of changes

• In wait_lsn, return WaitLsnError::Cancelled rather than BadState when the state is Stopping
• Replace PageReconstructError's Other variant with a specific BadState variant
• Avoid returning anyhow::Error from get_ready_ancestor_timeline -- this was only used for the case where there was no ancestor. All callers of this function had implicitly checked that the ancestor timeline exists before calling it, so they can pass in the ancestor instead of handling an error.

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]

3150 tests run: 3017 passed, 0 failed, 133 skipped (full report)

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Code coverage* (full report)

• functions: 31.4% (6489 of 20660 functions)
• lines: 48.4% (50225 of 103735 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
8a4dccc at 2024-05-30T09:29:05.268Z :recycle:}

[koivunej]

I don't see how this could be worse, but I am unsure how much better this is due to our too many checks at different levels and obfuscated error types.