BUG/MEDIUM: stconn: Report error on SC on send if a previous SE error was set

BUG/MEDIUM: stconn: Report error on SC on send if a previous SE error was set

When a send on a connection is performed, if a SE error (or a pending error)
was already reported earlier, we leave immediately. No send is performed.
However, we must be sure to report the error at the SC level if necessary.
Indeed, the SE error may have been reported during the zero-copy data
forwarding. So during receive on the opposite side. In that case, we may
have missed the opportunity to report it at the SC level.

The patch must be backported as far as 2.8.