Add explanation string to logic exceptions

[R-Goc]

I was trying out this library and made some logic bug (still not sure what at the time of writing). But all the information I get is logic error. For example in this code in parser.cpp:

    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();

In all these cases it would be better to instead of having a comment propagate that information into the exception. That way the error actually makes some sense and is helpful to figuring out what is wrong. This either needs changing detail::throw_logic_error or a new overload.

[ashtum]

@R-Goc, the serializer and parser are currently being redesigned, this issue along with many others has been addressed in the new interface. we'd definitely benefit from your input on the new design.

You can find the redesign documentation in these PRs:

• Serializer redesign doc #289
• Parser redesign doc #286

The full documentation is also available here (its temporary location until merged into http):

• https://develop.burl.cpp.al/burl/reference/boost/burl/serializer.html
• https://develop.burl.cpp.al/burl/reference/boost/burl/parser.html