dymoprint_gui does not work if printer is not on on startup

[tomek-szczesny]

The title says it all, it's easy to reproduce. A bit annoying when I type in the label design and only then I find out the printer wasn't on.
Instead of printing, a very long error message appears.

Perhaps the easiest workaround is to test printer presence on startup and exit with an angry pop-up message if not.
Another possibility is some sort of "green LED" in GUI that gets updated every 1 second, showing that the printer is detected.

I'm testing version 2.0.0 with PyUSB (#56).