bug#56845: unclear error message: No route to host

[Csepp]

```
guix pull -v3 --cores=1      
guix pull: error: failed to load '/home/raingloom/.config/guix/channels.scm': 
No route to host
```

My channels.scm mentions at least 3 hosts by name and indirectly uses
ci.guix.gnu.org via channel-with-substitutes-available.

It's quite possible this is more a Guile issue, but then again, Guix
should give a clear stack trace about which function the error
originates from.