For some reason the BorgExceptions from the backup thread don't get caught in
the main_window. Therefore I'm implementing the error handling directly in the
class for the moment. This will have the side effect that the error dialog will
pop up when there is a problem during a background backup.
However that is anyway something I thought about implementing. This way it's
just not properly implemented yet.
Exceptions which get raised in threads don't bubble up into the main code on
their own. So we either communicate them to the main loop or we handle them
directly in the thread. I'm going for the later for the moment because the
other way seems to be quite complex.