serial_link: Give up on IOError in ListenerThread.

This prevents spinning and eating memory when the USB port is unplugged with pyserial.
pylibftdi will still fail.
Code review fixes.

Print "Piksi disconnected" instead of exception and backtrace.
Don't handle error on reading sync byte in get_message, leave this
to ListenerThread.
Handle OSError as IOError as Piksi disconnected.
Merge pull request #145 from gsmcmullin/console_usb_unplug

serial_link: Give up on IOError in ListenerThread.

Closes #138