Add explanation string to logic exceptions #278

    auto
    prepare() ->
        mutable_buffers_type
    {
        nprepare_ = 0;

        switch(state_)
        {
        default:
        case state::reset:
            // reset must be called first
            detail::throw_logic_error();

        case state::start:
            // start must be called first
            detail::throw_logic_error();

        case state::header:
        {
            BOOST_ASSERT(
                m_.h_.size < cfg_->headers.max_size);
            std::size_t n = fb_.capacity();
            BOOST_ASSERT(n <= cfg_->max_overread());
            n = clamp(n, cfg_->max_prepare);
            mbp_[0] = fb_.prepare(n);
            nprepare_ = n;
            return mutable_buffers_type(&mbp_[0], 1);
        }

        case state::header_done:
            // forgot to call parse()
            detail::throw_logic_error();