MINOR: h2: centralize the check for the half-closed(remote) streams

MINOR: h2: centralize the check for the half-closed(remote) streams

RFC7540#5.1 is pretty clear : "any frame other than WINDOW_UPDATE,
PRIORITY, or RST_STREAM in this state MUST be treated as a connection
error of type STREAM_CLOSED". Instead of dealing with this for each
and every frame type, let's do it once for all in the main demux loop.