validate case index domains

http://manage.dimagi.com/default.asp?120127

@dannyroberts 
@snopoke I tried testing this locally by creating a case referencing a valid index in its own domain and submitting that. I still get the error response for bad domain. It looks like the main `case.domain` is `None` but the referenced case domain is set. I think this needs to be tested more (going to close for now). One workaround would be to also check if `case.domain` is set, but then we don't catch errors on create.