pageserver: don't squash all image layer generation errors into anyhow::Error

[jcsp]

Problem

CreateImageLayersError and CompactionError had proper From implementations, but compact_legacy was explicitly squashing all image layer errors into an anyhow::Error anyway.

This led to errors like:

 Error processing HTTP request: InternalServerError(timeline shutting down

Stack backtrace:
   0: <<anyhow::Error as core::convert::From<pageserver::tenant::timeline::CreateImageLayersError>>::from as core::ops::function::FnOnce<(pageserver::tenant::timeline::CreateImageLayersError,)>>::call_once
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/ops/function.rs:250:5
   1: <core::result::Result<alloc::vec::Vec<pageserver::tenant::storage_layer::layer::ResidentLayer>, pageserver::tenant::timeline::CreateImageLayersError>>::map_err::<anyhow::Error, <anyhow::Error as core::convert::From<pageserver::tenant::timeline::CreateImageLayersError>>::from>
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/result.rs:829:27
   2: <pageserver::tenant::timeline::Timeline>::compact_legacy::{closure#0}
             at pageserver/src/tenant/timeline/compaction.rs:125:36
   3: <pageserver::tenant::timeline::Timeline>::compact::{closure#0}
             at pageserver/src/tenant/timeline.rs:1719:84
   4: pageserver::http::routes::timeline_checkpoint_handler::{closure#0}::{closure#0}

Closes: #7861

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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Flaky tests (2)

Postgres 16

• test_pageserver_restarts_under_worload: release

Postgres 14

• test_pageserver_restarts_under_worload: release

Code coverage* (full report)

• functions: 31.4% (6530 of 20798 functions)
• lines: 48.3% (50421 of 104318 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
15b0d16 at 2024-06-03T13:30:20.053Z :recycle:}