BUG/MEDIUM: mux-h2: make sure to always report pending errors to the stream

BUG/MEDIUM: mux-h2: make sure to always report pending errors to the stream

Some stream parsing errors that do not affect the connection result in
the parsed block not being transferred from the rx buffer to the channel
and not being reported upstream in rcv_buf(), causing the stconn to time
out. Let's detect this condition, and propagate term flags anyway since
no more progress will be made otherwise.

This should be backported at least till 3.2, probably even 2.8.