Specify whether to throw on bad read in Mixer and WaveTrackCache...

... Do throw when exporting or mixing-and-rendering; don't if playing back or
drawing a spectrogram, but then just use zeroes.

src/export/Export.cpp

@@ -246,6 +246,8 @@ std::unique_ptr<Mixer> ExportPlugin::CreateMixer(const WaveTrackConstArray &inpu
 {
    // MB: the stop time should not be warped, this was a bug.
    return std::make_unique<Mixer>(inputTracks,
+                  // Throw, to stop exporting, if read fails:
+                  true,
                   Mixer::WarpOptions(timeTrack),
                   startTime, stopTime,
                   numOutChannels, outBufferSize, outInterleaved,