Example abnf2pegtl2.cpp does not throw custom errors.

[engelant]

I tried out the abnf2pegtl2.cpp example after I couldn't find out at which point the error_control is passed trough the tao::pegtl::parse_tree::parse to tao::pegtl::parse, and it seems it just isn't. Therefor custom error messages are not thrown.

[d-frey]

True. I will enhance parse_tree::parse to accept a base class for the internally generated control class.