Resolve undetected or infinitely-retried socket-send failures

My hub (1:218/700) is currently having what appears to be a TCP/IP
connectivity issue that was resulting in infinite "Send failure"
log messages and "We got an M_EOB, but there are still N files pending M_GOT"
log messages.

I first added better socket-send failure detection (checking return value of
sendCmd() and sendData() where needed) and then noticed that failure to send
a file was not detected (the sending.file.position is advanced even if
sendData() fails), so now handling that condition too.

Also added more diagnostics around socket-send failures (is socket writable?)
in this particular case, the socket is not writable and socket-send is
returning 0.