Add .find! methods to system unit modules

The reasoning behind this is:
If you can't handle any `nil` at the place of calling, use the finder with exclamation mark; `SystemdSocketNotFound` or `SystemdServiceNotFound` exception will be raised.

```ruby
 socket = Yast::SystemdSocket.find!('IcanHasCheez')
# => Yast::SystemdSocketNotFound: Socket unit 'IcanHasCheez' not found
```