frontend: don't traceback for "module exists" error
    
    We need to rollback() the session in case of exception, otherwise we
    end-up with:
    coprs.exceptions.DuplicateException: Module testmodule-beakertest-20210824084933 already exists
    During handling of the above exception, another exception occurred:
    ...
    DETAIL:  Key (copr_id, name, stream, version)=(44610, testmodule, beakertest, 20210824084933) already exists.
    Seems like some implicit flush is causing this.