Prior to the change the watcher index listener didn't implement the
`postIndex(ShardId, Engine.Index, Engine.IndexResult)` method. This
caused document level exceptions like VersionConflictEngineException
to be ignored. This commit fixes this.
The watcher indexing listener did implement the `postIndex(ShardId, Engine.Index, Exception)`
method, but that only handles engine level exceptions.
This change also unmutes the SmokeTestWatcherTestSuiteIT#testMonitorClusterHealth test again.
Relates to #32299