Improve error for structured output backend selection (#16717)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/v1/engine/processor.py

@@ -155,10 +155,13 @@ class Processor:
                              "supported in V1.")
         if params.guided_decoding.backend:
             if params.guided_decoding.backend != engine_level_backend:
-                raise ValueError("Request-level structured output backend "
-                                 "must match engine-level backend. "
-                                 f"{params.guided_decoding.backend}"
-                                 f" != {engine_level_backend}")
+                raise ValueError(
+                    "Request-level structured output backend selection is no "
+                    "longer supported. The request specified "
+                    f"'{params.guided_decoding.backend}', but vLLM was "
+                    f"initialised with '{engine_level_backend}'. This error "
+                    "can be resolved by removing backend selection from the "
+                    "request.")
         else:
             params.guided_decoding.backend = engine_level_backend