Salt provisioner missing error messages in localization file

The salt provisioner doesn't seem to have the error messages in the localization file. So you get an error about a missing translation key instead of a configuration problem in your provisioner.

```
salt:
* translation missing: en.salt.missing_key
```

I added the missing text values to the localization and fixed up the provisioner so the translation keys and error namespace is more consistent with the other provisioners.