Fix QString argument substitution

QString replaces only %1,%2 with arguments.
Add warning if bsdtar is absent

If zeal cannot launch bsdtar it loops total download progress bar
endlessly, which looks like it hangs. User must be warned if something
goes wrong, so I added QMessageBox to say about this.

Also, 'program' variable not changed anywhere and can be declared as
constant.
More helpful message about bsdtar missing