yacc.c: make sure we properly propagated the user's number for error

* data/skeletons/yacc.c (YYERRCODE): Be truthful.
* tests/input.at (Redefining the error token): Check that.

data/skeletons/yacc.c

@@ -673,7 +673,7 @@ static const ]b4_int_type_for([b4_toknum])[ yytoknum[] =
 
 /* Error token number */
 #define YYTERROR        1
-#define YYERRCODE       256
+#define YYERRCODE       ]b4_symbol(1, user_number)[
 
 ]b4_locations_if([[
 ]b4_yylloc_default_define[