Propagate the player error to ExoPlayerTestRunner

In a test run where no exceptions were thrown on the main thread and the test
did not time out, exceptions from onPlayerError were not correctly propagated to
the test thread (handleException would be called with null).

Fix ExoPlayerTestRunner.onPlayerError to propagate the actual exception from the
player.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176825907

testutils/src/main/java/com/google/android/exoplayer2/testutil/ExoPlayerTestRunner.java

@@ -567,7 +567,7 @@ public final class ExoPlayerTestRunner extends Player.DefaultEventListener {
 
   @Override
   public void onPlayerError(ExoPlaybackException error) {
-    handleException(exception);
+    handleException(error);
   }
 
   @Override