minorbug(more): when using /var/run/snapd.socket, the error shown as unknown

Hi, uutils mainteners

our test confirm a minor issue

```css
relunsec@relunsec:~/software/coreutils/target/debug$ ./more /var/run/snapd.socket
more: cannot open '/var/run/snapd.socket': uncategorized error
```

the error is uncategorized