Extracting information from Content warning failures

[fdalvi]

Something to think about later, but tasks such as Sentiment/Adult (and perhaps quite a few others) can use a "content warning" failure as a signal to assign a label (I would think "ADULT") instead of assigning a random label; I've merged the task in for now, perhaps we can discuss and think for after EMNLP.

@firojalam @hmubarak