Fix potential cause of infinite-spinner-without-failure.

It looks like for the case of self-contained media segments,
it's possible to get stuck without failure in the case that
the load fails having loaded less than the length of the init
data.

library/src/main/java/com/google/android/exoplayer/chunk/ChunkSampleSource.java

@@ -318,6 +318,9 @@ public class ChunkSampleSource implements SampleSource, Loader.Listener {
     }
 
     if (!mediaChunk.prepare()) {
+      if (currentLoadableException != null) {
+        throw currentLoadableException;
+      }
       return NOTHING_READ;
     }