allow pyslk function to handle exceptions

I think, we should have the possiblitiy to let a pyslk command handle exceptions if neccessary, e.g., have something like run_slk(slk_call, handle_exception=False) and return stderr. The reason is, e.g., that checksum should return None in the case that the checksum does not yet exist instead of raising and exception. That should be handled in the individual functions if this makes sense.