backend: indefinite loop fix for missing results.json
    
    This shouldn't actually normally happen, but just in case (the problem
    happened once when we had an old copr-rpmbuild package instlaled on
    builders).  The error output would be:
    
    Backend process error: Error while collecting built packages for 2134118-fedora-rawhide-x86_64:
    Worker failed build, took 116.72690773010254
    Retry request #1 on https://copr-fe-dev.cloud.fedoraproject.org/backend/update/: Request server error on https://copr-fe-dev.cloud.fedoraproject.org/backend/update/: 500 INTERNAL SERVER ERROR
    Retry request #2 on https://copr-fe-dev.cloud.fedoraproject.org/backend/update/: Request server error on https://copr-fe-dev.cloud.fedoraproject.org/backend/update/: 500 INTERNAL SERVER ERROR
    ...
    
    While I'm on it, let's move the _parse_results on a slightly better
    place where the exceptions aren't ignored.