While buffering check for errors even if renderes are ready.

On an edge case, player may get stuck when the renderers are ready but the buffer doesn't get full enough because of a fatal error in data source. An example state can be created by starting a live DASH stream and switching between normal and slow network connections.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129084824

library/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -470,6 +470,12 @@ import java.util.ArrayList;
       }
     }
 
+    if (state == ExoPlayer.STATE_BUFFERING) {
+      for (Renderer renderer : enabledRenderers) {
+        renderer.maybeThrowStreamError();
+      }
+    }
+
     handler.removeMessages(MSG_DO_SOME_WORK);
     if ((playWhenReady && state == ExoPlayer.STATE_READY) || state == ExoPlayer.STATE_BUFFERING) {
       scheduleNextOperation(MSG_DO_SOME_WORK, operationStartTimeMs, RENDERING_INTERVAL_MS);