iter_study_objs() will crash if study has been deleted

[pmidford]

I copied and modified the ott_status_report script to generate historical statistics from the phylesystem-1 shard. I encountered a problem where the iter_study_objs() method crashed with a can't open file error pointing to a nexson in the local copy of the shard. The file was definitely there - I think it may be related to the study having been deleted at some point, though it somehow still turned up in the list of study ids. I worked around the problem using iter_study_filepaths and os.path_isfile, but I'm concerned that there maybe something funny and git-related that I don't understand here.