LUCENE-4903: Add AssertingScorer.


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1466709 13f79535-47bb-0310-9956-ffa450edef68
fix test to stop if indexing threads hits an exception

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1466714 13f79535-47bb-0310-9956-ffa450edef68