c: don't emit an error message when the scanner returns YYERRCODE

* data/skeletons/yacc.c (yyparse): When the scanner returns YYERRCODE,
go directly to error recovery (yyerrlab1).
However, don't keep the error token as lookahead, that token is too
special.
* data/skeletons/lalr1.cc: Likewise.

* examples/c/bistromathic/parse.y (yylex): Use that feature to report
nicely invalid characters.
* examples/c/bistromathic/bistromathic.test: Check that.
* examples/test: Neutralize gratuitous differences such as rule
position.

* tests/calc.at: Check that case in C only.
The other case seem to be working, but that's an illusion that the
next commit will address (in fact, they can enter endless loops, and
report the error several times anyway).

data/skeletons/lalr1.cc

@@ -981,6 +981,16 @@ b4_dollar_popdef])[]dnl
       }
     YY_SYMBOL_PRINT ("Next token is", yyla);
 
+    if (yyla.kind () == ]symbol_kind::b4_symbol(1, kind)[)
+    {
+      // The scanner already issued an error message, process directly
+      // to error recovery.  But do not keep the error token as
+      // lookahead, it is too special and may lead us to an endless
+      // loop in error recovery. */
+      yyla.type = ]symbol_kind::b4_symbol(2, kind)[;
+      goto yyerrlab1;
+    }
+
     /* If the proper action on seeing token YYLA.TYPE is to reduce or
        to detect an error, take that action.  */
     yyn += yyla.kind ();

data/skeletons/yacc.c

@@ -1770,6 +1770,17 @@ yyread_pushed_token:]])[
       yytoken = ]b4_symbol(0, [kind])[;
       YYDPRINTF ((stderr, "Now at end of input.\n"));
     }
+  else if (yychar == ]b4_symbol(1, [id])[)
+    {
+      /* The scanner already issued an error message, process directly
+         to error recovery.  But do not keep the error token as
+         lookahead, it is too special and may lead us to an endless
+         loop in error recovery. */
+      yychar = ]b4_symbol(2, [id])[;
+      yytoken = ]b4_symbol(1, [kind])[;]b4_locations_if([[
+      yyerror_range[1] = yylloc;]])[
+      goto yyerrlab1;
+    }
   else
     {
       yytoken = YYTRANSLATE (yychar);