[Feature]: Improve DCP error messages

[LucasWilkinson]

🚀 The feature, motivation and pitch

Currently if a backend doesn't support DCP we get the following error message

AssertionError: DCP requires attention impls to return the softmax lse for decode, but the impl FlashInferImpl does not return the softmax lse for decode.

It would good to suggest to the user to try an alternative backend using VLLM_ATTENTION_BACKEND

Alternatives

No response

Additional context

No response

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[WorldExplored]

Working on it.