Warning user that changing update mode from "patterns" to "update only installed rpms" might lead into broken system.

- Only if changing from "patterns" to "update only installed rpms"
- Only if "patterns" were the default method
- BNC#873122
- Tested in modified build 271 and works ;)

Yes, the code is not very nice and might still need some refactoring.
That's on my TODO list.