Method exception_message

Source/external/json.hpp:12564–12591

	}

	std::string exception_message(const token_type expected, const std::string& context)
	{
		std::string error_msg = "syntax error ";

		if (!context.empty())
		{
			error_msg += concat("while parsing ", context, ' ');
		}

		error_msg += "- ";

		if (last_token == token_type::parse_error)
		{
			error_msg += concat(m_lexer.get_error_message(), "; last read: '",
								m_lexer.get_token_string(), '\'');
		}
		else
		{
			error_msg += concat("unexpected ", lexer_t::token_type_name(last_token));
		}

		if (expected != token_type::uninitialized)
		{
			error_msg += concat("; expected ", lexer_t::token_type_name(expected));
		}

		return error_msg;
	}

  private:
	/// callback function