fix(pageserver): receive body error now 500

[skyzh]

Problem

close #8903

In #8903 we observed JSON decoding error to have the following error message in the log:

Error processing HTTP request: Resource temporarily unavailable: 3956 (pageserver-6.ap-southeast-1.aws.neon.tech) error receiving body: error decoding response body

This is hard to understand. In this patch, we make the error message more reasonable.

Summary of changes

• receive body error is now an internal server error, passthrough the reqwest::Error (only decoding error) as anyhow::Error.
• instead of formatting the error using to_string, we use the alternative anyhow::Error formatting, so that it prints out the cause of the error (i.e., what exactly cannot serde decode).

I would expect seeing something like error receiving body: error decoding response body: XXX field not found after this patch, though I didn't set up a testing environment to observe the exact behavior.

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]

4968 tests run: 4804 passed, 0 failed, 164 skipped (full report)

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Flaky tests (4)

Postgres 17

• test_scrubber_physical_gc[4]: release-x86-64, debug-x86-64

Postgres 16

• test_neon_cli_basics: release-arm64

Postgres 15

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64

Code coverage* (full report)

• functions: 31.9% (7424 of 23301 functions)
• lines: 49.8% (59741 of 119910 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
3b39f86 at 2024-09-19T20:07:17.798Z :recycle:}

[skyzh]

added a commit to only return 500 for decoding errors; for other reqwest errors, also log the error source