Remove "state" from "Invalid state parameter" error message

- It's confusing to see this from inside an anonymous function. Actually,
  maybe it should be reworded altogether?

src/thingdef/thingdef_states.cpp

@@ -583,7 +583,7 @@ FxVMFunctionCall *ParseAction(FScanner &sc, FState state, FString statestring, B
 		}
 		return call;
 	}
-	sc.ScriptError("Invalid state parameter %s\n", sc.String);
+	sc.ScriptError("Invalid parameter '%s'\n", sc.String);
 	return NULL;
 }