fix(layer): error out early if layer path is non-file

[koivunej]

In an earlier PR #5743 (comment) I added a FIXME and there's a simple solution suggested by @jcsp, so implement it. Wondering why I did not implement this originally, there is no concept of a permanent failure, so this failure will happen quite often. I don't think the frequency is a problem however.

Sadly for std::fs::FileType there is only decimal and hex formatting, no octal.

[github-actions]

2340 tests run: 2224 passed, 0 failed, 116 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_restarts_frequent_checkpoints: release

Postgres 15

• test_competing_branchings_from_loading_race_to_ok_or_err: debug
• test_ts_of_lsn_api: debug
• test_empty_tenant_size: debug

Code coverage (full report)

• functions: 53.3% (8644 of 16206 functions)
• lines: 81.4% (50090 of 61513 lines)

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_{The comment gets automatically updated with the latest test results
7b56c21 at 2023-11-02T11:16:20.029Z :recycle:}

[jcsp]

Looks fine, presuming the e032bb9 commit is the only one that this PR is about

[koivunej]

Looks fine, presuming the e032bb9 commit is the only one that this PR is about

I forgot I had already created a PR on this so you probably reviewed this while a parent PR was merged and before I pushed a rebase (right now). It was supposed to be two commits, one to treat it as an error, second to remove the fixme (it is that way now).

Thanks for review.

[koivunej]

Failures on Postgres 15

• test_ts_of_lsn_api: debug

Created #5768.