pageserver: fix a 500 during timeline creation + shutdown

[jcsp]

Problem

The test_create_churn_during_restart test fails if timeline creation calls return 500 errors (because the API shouldn't do it), and it's sometimes failing, for example:
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-10256/12582034135/index.html#/testresult/3ce2e7045465012e

Summary of changes

• Avoid handling UploadQueueShutDownOrStopped case as an Other (i.e. 500)

[github-actions]

7095 tests run: 6797 passed, 0 failed, 298 skipped (full report)

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Code coverage* (full report)

• functions: 31.3% (8403 of 26881 functions)
• lines: 48.0% (66688 of 139075 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
9ce7646 at 2025-01-02T15:58:55.069Z :recycle:}