BUG/MEDIUM: mux-h2: report asynchronous errors in h2_wake_some_streams()

BUG/MEDIUM: mux-h2: report asynchronous errors in h2_wake_some_streams()

This function is called when dealing with a connection error or a GOAWAY
frame. It used to report a synchronous error instead of an asycnhronous
error, which can lead to data truncation since whatever is still available
in the rxbuf will be ignored. Let's correctly use CS_FL_ERR_PENDING instead
and only fall back to CS_FL_ERROR if CS_FL_EOS was already delivered.

No backport is needed.