pg.{set,read}_config rely on having a non-broken version of postgres

So if you, for instance, run:

```
fab pg.set_config:shared_preload_libraries,'citus\,cstore_fdw' pg.restart
```

Without installing cstore first there's no easy way to fix the state you're now in. Running another `pg.set_config` will fail because it uses `ALTER SYSTEM` rather than edit the file directly.