Fix crash loop: only exit on CRITICAL, not ERROR

[hbd]

Summary

• Change sys.exit(1) trigger from ERROR level to CRITICAL level in DelayedRichHandler
• Prevents crash loops when users provide incorrect configuration (e.g., wrong model name causing 404s)

Problem

Previously, any ERROR log would trigger sys.exit(1), causing:

• Immediate crash on first 404 (wrong model name)
• Crash loops when the benchmark runner retries
• No useful output for debugging

Solution

Only exit on CRITICAL logs. Recoverable errors like HTTP 404s are logged but the benchmark continues, reporting 0 successful requests which correctly indicates the failure without crashing.

Test plan

• Run benchmark with incorrect model name - should complete with 0 requests instead of crashing
• Run benchmark with correct config - should work as before

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