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

[sapphi-red]

Description

#7398 made Vite to return 500 when proxy had an error. But this is not possible if response is already sent.
This PR will make Vite to only return 500 if response is not sent.

fixes #9172

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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