Resolve some board connection errors

[gillens]

Resolves #29, starts work on #35

• First and third commits just change logging a bit so that it prints the full error if it actually can't connect but doesn't try to connect when there's nothing there.
• Second commit isn't anything new, it just reverts back to some code that I think got deleted accidentally during a refactor. Tries to connect just once instead of twice and updates selection dropdown properly. This means that it's no longer necessary to restart PSplot if the device is plugged in after starting PSplot. Also possible to switch between devices.

[Jerzeek]

Tested it and it works perfectly! Awesome work @gillens