More accurate ResponseBodyResultHandler error message

spring-web-reactive/src/main/java/org/springframework/reactive/web/dispatch/method/annotation/ResponseBodyResultHandler.java

@@ -142,7 +142,7 @@ public class ResponseBodyResultHandler implements HandlerResultHandler, Ordered
 			return response.writeWith(outputStream);
 		}
 		return Publishers.error(new IllegalStateException(
-		  "Return value type not supported: " + returnType));
+		  "Return value type '" + returnType.getParameterType().getName() + "' with media type '" + mediaType + "' not supported"  ));
 	}
 
 	private MediaType resolveMediaType(ServerHttpRequest request) {