[V1] Improve Error Message for Unsupported Config (#12535)

Co-authored-by: Michael Goin <michael@neuralmagic.com>

vllm/platforms/cuda.py

@@ -120,13 +120,18 @@ class CudaPlatformBase(Platform):
         if parallel_config.worker_cls == "auto":
             if scheduler_config.is_multi_step:
                 if envs.VLLM_USE_V1:
-                    raise NotImplementedError
+                    raise NotImplementedError(
+                        "Multi-step scheduling is not supported (and not "
+                        "needed) on VLLM V1. Please launch without "
+                        "--num-scheduler-steps.")
                 else:
                     parallel_config.worker_cls = \
                         "vllm.worker.multi_step_worker.MultiStepWorker"
             elif vllm_config.speculative_config:
                 if envs.VLLM_USE_V1:
-                    raise NotImplementedError
+                    raise NotImplementedError(
+                        "Speculative decoding is not yet supported on VLLM V1."
+                    )
                 else:
                     parallel_config.worker_cls = \
                         "vllm.spec_decode.spec_decode_worker.create_spec_worker"