Fix async client swallowing backend errors during flush

[ArnavBalyan]

Changes

• Async client flush catches HTTPError but the call uses aiohttp, so the except never matched. Backend errors propagated up unhandled and killed the worker task.
• Add a raise so failures actually surface.
• Switch to except aiohttp.ClientResponseError, matching every other async method in this file.

How I tested this

• UT

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.