add error and warning on new factor levels for prediction

[jakob-r]

General problem: New factor levels appeared during prediction.

Behaviour: fix.factors.prediction = TRUE will convert new factors to NAs

Concrete problem: There is no warning that this happens.

More concrete problem: Some learners (eg. ksvm) will just drop rows with NAs in prediction, leading to fewer predictions than in newdata. (#2771)

Less heavy problem: Some learners (eg. ranger) will complain about NAs, but the user does not now where these are comming from (#2611)

Solution: Create a warning, when NAs are created, throw an error if prediction has not the right number of items. Both in combination will hopefully give the user a clue of what is happening.

Also related: #2602

[pat-s]

Thanks @jakob-r! Could you also add a descriptive NEWS entry for this?

[jakob-r]

done.

[pat-s]

Thanks. LGTM.