Give better error on blank entity id

Previously was a pretty useless error in the logs: http://logs.internal.dimagi.com:9000/commcare-hq-dev-team/hq-production/group/78612
this seems like the simplest, though maybe not the most correct/perfect change e.g. if something in `wrap_commtrack_case` raises a different `KeyError`. will merge since it's not super likely, but feel free to fix if you agree.