fix: return 500 on proxy error only if possible (fixes #9172)

[sapphi-red]

Description

Revises #9175.
I should have used res.headersSent.
I'm not sure if I should check res.writableEnded too. So I left that.

fixes #9172

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
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