Comment 8 for bug 1740697

This commit slightly changed the behavior of CachingReader::read() by propagating cache misses properly to the caller instead of implicitly filling the remaining sample buffer with silence:

844cf3505c6512e5c62d8c01a2b99f18464cb8fc

Test results should be unaffected. Now the buffer will be filled with silence in one of the EngineBufferScale classes and tests will fail if the decoded sample data is not available in time.

The CachingReader API has no proper way to propagate errors. Currently both permanently unreadable and temporarily unavailable sample data cannot be distinguished by the caller. I already mentioned that this layer above the SoundSources is the next candidate for an overhaul, 2nd round.