Erroneous condition:
===================
If the track selection contains a subset of the available variants in the
master playlist, but only the selected variants return 404, the playlist
tracker will never propagate the error.
Fix:
====
The Chunk source will propagate the playlist load error if no more
alternative playlists are available (because all are already blacklisted).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190624484
