Exit with error when QuiverQuant fetch returns no data

[AlexCatarino]

Summary

• Detect QuiverQuant connection failures by treating an empty per-ticker accumulator at Flush() time as a hard failure (logs an error and returns false).
• Wire Program.Main to propagate the Flush() result so the process exits with code 1 instead of silently succeeding when the API is unreachable.
• Applies to the CNBC and InsiderTrading downloaders, whose multi-date loops previously logged per-date errors but always reported success overall.

Test plan

• Run the CNBC downloader with a valid API key and confirm successful runs still exit 0 and write per-ticker files.
• Run the CNBC downloader with the API unreachable (bad key / blocked host) and confirm the process exits 1 with the new Flush(): No data accumulated error.
• Repeat both checks for the InsiderTrading downloader.