- Add a top level failure type (source/renderer/unexpected),
and convenience methods for retrieving the underlying cause
without needing to cast.
- Also add renderer index in the case of renderer failures.
- setIndex/getIndex is a little . . . unclean, but alternatives
involve either having the top line of the stack trace be a
non-interesting line, or loads of try/catch blocks in
ExoPlayerImplInternal.
Issue: #777
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