Fixing error message issues.

I fixed several issues in grml2usb which concerns the error handling. So in general, I retained the exceptions for the control flow, but any non-fatal error message is logged using the logging module. Once a fatal error occurs, an Exception is triggered and the main method catches the error. This error gets logged with the logging module at level "critical".

I did some tests using a mockup version, but I would be happy if you can apply some tests yourself if you consider merging this into some stable branch.