Return NULL instead of throwing when a genome doesn't exist
Return NULL instead of throwing when a genome doesn't exist

Sort of surprising behavior, but apparently this is what's supposed to
happen.

glennhickey/hal,glennhickey/hal,glennhickey/hal,glennhickey/hal,glennhickey/hal,glennhickey/hal,glennhickey/hal,glennhickey/hal