Split error codes in two

[Martin Thomson <[e-mail]>]

Input sought.

I've a proposal in that splits the error code space:

   https://github.com/quicwg/base-drafts/pull/722

The basic idea here - one that I believe that we have agreement on,
but would like to confirm - is that only applications should be
cancelling streams.  In short: if the transport kills a stream that
the application was relying on, the whole application protocol state
becomes indeterminate.

In order to ensure no possibility of confusion, I think that it is
cleaner to have transport errors and application errors.  The above PR
makes that split.

It's a pretty clean cut, but there are a few costs: CONNECTION_CLOSE
needs to be split because both application and transport might need to
kill a connection.  The NO_ERROR code is then duplicated.  There is
surprisingly little need for overlap on other error codes.


In doing this, I realized two further things:

1. STOP_SENDING is a purely application-layer construct, but it's a
transport-layer frame.  That makes it much harder to manage with this
change.  Moving it to HTTP makes a lot of sense (to me).

   https://github.com/quicwg/base-drafts/pull/759

2. We are now nudging 20 error codes (plus the per-frame-type ones).
4 billion might be a little more space than we need, so halving the
number of bits with give error cords seems reasonable:

   https://github.com/quicwg/base-drafts/pull/723