change ItemNotFoundErrors to log.warning on bringup

@jarv 
@cpennington 

Apparently we get tons of these when we start the LMS and load all the courses. Lowering the logging level to clean up the logs a little.
It might be worth putting in a tag for the block type of `self` as well, so that we can see if there are any correlations on that end of the parent/child association.