Prevent false version mismatches when saving annotations

[fm3]

During fossildb restart some save requests were wrongly answered with 409 CONFLICT (version mismatch, expected 1 got 313), when they should have been answered with 400 (or 500, or anything that doesn’t prevent the frontend from retrying). This was caused by falling back to 0 not only if the error cause was no-such-element (in which case 0 is correct), but always. This PR adds this distinction.

URL of deployed dev instance (used for testing):

• dev instance testing is difficult since we can’t kill the fossildb there.

Steps to test:

• create tracing, save some versions (should work with old and new tracings)
• kill fossildb (or restart server without fossildb, e.g. comment it out in tools/proxy/proxy.js), but keep tracing tab open. saving should now fail, but not with error 409, frontend should say “retrying”

Issues:

• fixes version mismatch during fossildb restart #4590

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• Updated (unreleased) changelog
• Needs tracingstore update after deployment
• Ready for review

[youri-k]

LGTM