Fix issue with replication of child datasets silently failing

As reported by David Samms dsamms@nw-ds.com, when replicating with -r, if replication of one of the child datasets failed, subsequent checks did not occur for that child dataset.

% zap rep -v -r zroot/ROOT

## Replication of zroot/ROOT completed, but replication of zroot/ROOT/default
   failed.

% zap rep -v -r zroot/ROOT

## The local and remote snapshots matched for zroot/ROOT and the snapshots
   for zroot/ROOT/default were not compared, so no errors were reported.

Also, proceed with a warning for untested OSes instead of bailing out.