[FTS] moss OnError()/Log() callbacks to propagate to cbft

As noted by Marty, moss has an optional OnError() callback API.

The bleve/index/store/moss adapter probably needs to use that.

And cbft, in turn, probably needs some changes to propagate errors (log them?).

Similarly, I think moss has a Log() callback API that might also be utilized.

This jira ticket is a reminder to track this error propagation down.