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

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
(cherry picked from commit d5ca4ed76bc83af27f2ade59ce49e35750aa4177)