Improve json parsing / validation error reporting

A long standing issue... A typical error report for parameters encoded in JSON looks like this

Error: ./boost/json/impl/parse.ipp(63): Throw in function boost::json::value boost::json::parse(string_view, storage_ptr, const parse_options&)
Dynamic exception type: boost::wrapexcept<boost::system::system_error>
std::exception::what: extra data [boost.json:2 at ./boost/json/impl/parser.ipp:110:9 in function 'std::size_t boost::json::parser::write(const char*, std::size_t, boost::system::error_code&)']

and is pretty much useless.