Safety: verify pre/post-assessment references are valid.
Safety: verify pre/post-assessment references are valid.

Aparna was having problems with a course that had deleted
an assessment that was referred to from a unit.  (This was
done w/ a code version before the existing fix to prevent
this)
	Change on 2014/05/28 by mgainer <mgainer@google.com>
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=68116566

andela-angene/coursebuilder-core,andela-angene/coursebuilder-core,andela-angene/coursebuilder-core,andela-angene/coursebuilder-core