Fixed bad logical bug from Acorn's MainState.load rollback

The major bug was the logical not in MainState.load rollback which
caused the database revisioning to be started from 0 when the database
was *not* empty. It should have been the other way around.

Also cleaned up the database head.state validation code by not using
exceptions for flow control in validating head.state files.

refs #7124

Change-Id: I7cd57fa73d39a637c71159df63566aed5063fc40