safekeeper: downgrade interpreted reader errors

[erikgrinaker]

Problem

This critical! could fire on IO errors, which is just noisy.

Resolves #11027.

Summary of changes

Downgrade to error, except for decode errors. These could be either data corruption or a bug, but seem worth investigating either way.

[github-actions]

7744 tests run: 7365 passed, 0 failed, 379 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_s3_wal_replay[local_fs]: release-arm64-with-lfc

Postgres 14

• test_timeline_copy[500000]: release-x86-64-with-lfc

Code coverage* (full report)

• functions: 32.8% (8642 of 26363 functions)
• lines: 48.6% (73201 of 150514 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
d713047 at 2025-02-28T10:32:13.591Z :recycle:}