Improve exception reporting in MP engine

[vmarkovtsev]

"raise e from None" means changing the error bubble narrative from "another exception caused by e" to "exception e was raised and we forward it up"

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[njhill]

Thanks @vmarkovtsev. This is on the V0 path - could I ask which version of vLLM you are using and and whether you're using V0 mode intentionally and if so why?

[vmarkovtsev]

Sure, we are on the bleeding edge but with Neuron. On our custom Neuron backend which is a complete rewrite fwiw, but it shouldn't matter, as I understand.