Behavior after a failed call to cropTo()

[eladalon1983]

Consider a track cropped-to cropTarget1. Assume a valid cropTarget2 is produced to match element2, then element2 is removed from the DOM and eventually garbage-collected. If we now call track.cropTo(cropTarget2), it would return a rejected promise. But what should happen with track? Should it keep issuing frames cropped to cropTarget1, or should it be muted?

(FWIW, Chrome's current behavior is that the track no longer issues new frames. Up for debate if that's desirable.)