Added a ClusterHealthResponse.toString implementation for better error reporting in FullRollingRestartTests timeout failures.
Assert on failures in QueryRescorerTests
Make ContextIndexSearcher delegate more method calls to Engine.searcher().

When running tests, Engine.searcher() is going to be an AssertingIndexSearcher
so we definitely don't want to discard it. This commit fixes it as well as the
bugs it found.

Closes #3987