fix(proxy): better error when upstream proxy fails

[TheAlexLichter]

🔗 Linked issue

#722

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR should ensure that proxyRequests fail gracefully instead of leading to an unhandled node error crashing the server.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[TheAlexLichter]

Updated to sendProxy.

I think only we can assume only proxy had been failed and perhaps we can double check if there is not already set statusCode and message on error (that currently is being ingored)

You mean an existing one in event.node.statusCode / statusMessage or in the error directly? If the latter, which values should I check?

[pi0]

Thanks ❤️ (updated impl to still throw error to allow hooking into it but with createError)