if updateLog isn't in config, complain loudly if it's needed for SolrCloud

The updateLog can be commented out, and SolrCloud will only contain this error:

2019-07-03 16:15:55.907 ERROR (qtp214649627-92) [c:gettingstarted s:shard1 r:core_node4 x:gettingstarted_shard1_replica_n2] o.a.s.c.SyncStrategy No UpdateLog found - cannot sync

I suspect it's truly required in which Solr should complain more loudly about this – throw an exception. For single replica collections, I suppose it's fine (not an error?)? Granted then there could be an issue of adding replicas later (unless pull type; those are fine). Even if multi-replica SolrCloud can usefully work without an updateLog (news to me) then maybe we shouldn't log an error, although the lack of an updateLog would probably be an oversight that would be hard to track down the root cause of.

I wish the updateLog altogether could be more optional but that's another conversation.