core: Add RootDirectory= path to error message if directory does not exist

core: Add RootDirectory= path to error message if directory does not exist

A colleague reported when RootDirectory= does not exist, systemd reports an error like:
```
Failed to set up mount namespacing: No such file or directory
```

Unfortunately, with large spec files, it can be hard to diagnose which path systemd is talking
about. Thus, to make the error message more helpful and similar to mount error messages, we add
the root directory/image path into the error message like:
```
Failed to set up mount namespacing: /tmp/thisdoesnotexist: No such file or directory
```