[misc] add early error message for custom ops

[youkaichao]

reported by @joschu-anthropic

I think it might also solve #2021 (comment) , cc @yajuvendrarawat

[github-actions]

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[joschu-anthropic]

Looks good!
Maybe for clarity you could slightly revise to something like
"vLLM needs to register custom ops, but this version of pytorch does not support it -- chances are you are using..."

[joschu-anthropic]

LGTM!