prepare/nmstate.sh: properly exit setup on error
This typo fix make the test just switch from FAILED to SKIPPED. Proper fix would involve fixin installation, but make_rpm.sh is gone and the current installation methods documented by nmstate themselves[1] don't work:
- pip results in empty UNKNOWN-0.0.0 package
- installation of copr package makes libnmstate available however these
subsequent imports fail as well:
- from nmstatectl import nmstatectl
- from libnmstate.ifaces import BaseIface, and a lot of similar