postprocess: Make use of _prefix_error for format printing
postprocess: Make use of _prefix_error for format printing

Followup from https://github.com/projectatomic/rpm-ostree/pull/55

mdshuai/rpm-ostree,mdshuai/rpm-ostree,tmp-user/rpm-ostree,giuseppe/rpm-ostree,tmp-user/rpm-ostree,giuseppe/rpm-ostree,mdshuai/rpm-ostree,giuseppe/rpm-ostree,mdshuai/rpm-ostree,giuseppe/rpm-ostree,tmp-user/rpm-ostree,tmp-user/rpm-ostree