send_batch function cant handle socket err on win32

[JohnnanWang]

Hi, the send_batch function in lsquic_engine.c has some socket error handling according to errno it catched after packets_out. However, in send_packets_one_by_one function, socket error would not be assigned to error on win32, the errno would be zero on win32 even socket has some sending errors

[litespeedtech]

we are a linux shop, but pull requests for other platform are welcome.