sd-capture: More program UI cleanup

Add a progress bar, remove debugging print statements, prettify the
"Connect" screen.
sd-capture: More UI / code cleanup

This gives an error message if there is a network connection issue.
It also delays asking for the logfile until a connection is actually made.
mainwindow: Show textual string from network error

When there's a network error, show the actual error message, rather
than simply an error code.
mainwindow: Remove "unused variable" warning
networkconnection: Fix crash at startup on stale FIFO

If we connect to a Kovan that has stale NAND packets in its FIFO,
then it will immediately start sending those packets to us over the
network.  Rather than crash, simply ignore the packets until we've
actually started running a script and are ready to accept them.