Restore the "no map block registered" assertion when it makes sense
Restore the "no map block registered" assertion when it makes sense

Minor amendment to bd377ff -- if the view that triggered the index
update has no map block set, we should fail instead of just skipping
it, because that genuinely is an error (otherwise we'd go on to query
it and just return missing or obsolete data.)
For #438

alfared/couchbase-lite-ios,lordkev/couchbase-lite-ios,pegli/couchbase-lite-ios,brendand/couchbase-lite-ios,brendand/couchbase-lite-ios,amco/couchbase-lite-ios,brendand/couchbase-lite-ios,tomco/couchbase-lite-ios,couchbase/couchbase-lite-ios,amco/couchbase-lite-ios,amco/couchbase-lite-ios,Chavenay/couchbase-lite-ios,amco/couchbase-lite-ios,tomco/couchbase-lite-ios,Chavenay/couchbase-lite-ios,macoscope/couchbase-lite-ios,amco/couchbase-lite-ios,mz2/couchbase-lite-ios,couchbase/couchbase-lite-ios,tomco/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,lordkev/couchbase-lite-ios,pegli/couchbase-lite-ios,couchbase/couchbase-lite-ios,lordkev/couchbase-lite-ios,amco/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,couchbase/couchbase-lite-ios,macoscope/couchbase-lite-ios,mz2/couchbase-lite-ios,alfared/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios,amco/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,mz2/couchbase-lite-ios,pegli/couchbase-lite-ios,tomco/couchbase-lite-ios,pegli/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,brendand/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,alfared/couchbase-lite-ios,lordkev/couchbase-lite-ios,Chavenay/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,macoscope/couchbase-lite-ios,macoscope/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,brendand/couchbase-lite-ios,alfared/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios