Refactoring SuSEFirewallService - Firewall throws exception

- Mostly to throw an exception if required service is not found, till now
  it used to be just logged and thus was hard to find out what was wrong
- bnc#867377
- Also used in `CWMFirewallInterfaces` -- the most prominent place
- Changes file will be adjusted right before merging, I do not want to be merging conflicts in few days
- Full module cleanup hasn't been done, this is just too much work now