vectorized send() should handle EWOULDBLOCK

The vectorized write path in /util/net/sock.cpp only handles EAGAIN, unlike the non-vectorized path (which also handles EWOULDBLOCK). Both can receive it (from the SO_SNDTIMEO sockopt).

Failing to handle it means that socket timeouts will be sent up to the rest of the system as SocketException::SEND_ERROR, rather than SEND_TIMEOUT.