proxy: categorise new cplane error message

[conradludgate]

Problem

422 Unprocessable Entity: compute time quota of non-primary branches is exceeded being marked as a control plane error.

Summary of changes

Add the manual checks to make this a user error that should not be retried.

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]

2490 tests run: 2369 passed, 0 failed, 121 skipped (full report)

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Flaky tests (2)

Postgres 15

• test_compute_pageserver_connection_stress: debug
• test_sharding_split_smoke: debug

Code coverage* (full report)

• functions: 28.8% (7028 of 24429 functions)
• lines: 47.6% (43455 of 91330 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
6b01876 at 2024-03-08T12:28:42.120Z :recycle:}