BUG/MEDIUM: stream-int: Process connection/CS errors during synchronous sends

BUG/MEDIUM: stream-int: Process connection/CS errors during synchronous sends

If an error occurred on the connection or the conn-stream, no syncrhonous send
is performed. If the error was not already processed and there is no more I/O,
it will never be processed and the stream will never be notified of this
error. This may block the stream until a timeout is reached or infinitly if
there is no timeout.

Concretly, this bug can be triggered time to time with h2spec, running the test
"http2/5.1.1/2".

This patch depends on the commit 328ed220a "BUG/MINOR: stream-int: Process
connection/CS errors first in si_cs_send()". Both must be backported to 2.0 and
probably to 1.9. In 1.9, the code is totally different, so this patch would have
to be adapted.