c++: demonstrate custom error messages in the examples

Let's use c++/glr to demonstrate custom error messages in C++ (not
just in glr2.cc).

* examples/c++/glr/c++-types.yy (report_syntax_error): New.
* examples/c++/glr/c++-types.test: Adjust.
* examples/c/bistromathic/parse.y: Comment changes.
* tests/local.at (AT_YYERROR_DEFINE(c++)): Use a nicer way to print
the lookakead's name.