Add error message if track is added during transformation

PiperOrigin-RevId: 505670386

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/CompositeAssetLoader.java

@@ -153,7 +153,9 @@ import java.util.concurrent.atomic.AtomicLong;
       sampleConsumersByTrackType.put(trackType, sampleConsumer);
       sampleConsumersByTrackType.put(trackType, sampleConsumer);
       return sampleConsumer;
       return sampleConsumer;
     }
     }
-    return checkStateNotNull(sampleConsumersByTrackType.get(trackType));
+    return checkStateNotNull(
+        sampleConsumersByTrackType.get(trackType),
+        "The preceding MediaItem does not contain any track of type " + trackType);
   }
   }
 
 
   @Override
   @Override