Better error message for unprivileged bootc usroverlay

The error message for running `bootc usroverlay` as a non-root user does not make much sense.

```
$ bootc usroverlay
error: Read-only file system
$ sudo bootc usroverlay
Development mode enabled. A writable overlayfs is now mounted on /usr.
All changes there will be discarded on reboot.
```

Perhaps it can suggest running as the root user instead.

There are also two spaces after the first sentence in the usroverlay message. I'm not sure if that's intentional or not.