pageserver: improve debuggability of timeline creation failures during chaos testing

[jcsp]

Problem

We're seeing timeline creation failures that look suspiciously like some race with the cleanup-deletion of initdb temporary directories. I couldn't spot the bug, but we can make it a bit easier to debug.

Related: #11296

Summary of changes

• Avoid surfacing distracting ENOENT failure to delete as a log error -- this is fine, and can happen if timeline is cancelled while doing initdb, or if initdb itself has an error where it doesn't write the dir (this error is surfaced separately)
• Log after purging initdb temp directories

[github-actions]

7964 tests run: 7580 passed, 0 failed, 384 skipped (full report)

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Flaky tests (1)

Postgres 17

• test_compute_pageserver_connection_stress: release-x86-64-without-lfc

Code coverage* (full report)

• functions: 32.3% (8739 of 27024 functions)
• lines: 48.4% (74898 of 154749 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
6ba109d at 2025-03-18T18:46:03.376Z :recycle:}