guix pull errors

[Ekaitz Zarraga]

Hi,

I'm getting tons of guix pull errors these days.
They look like they are related with network issues because they
disappear if I insist (many times) and they appear in different
packages.

They look like this:

```
guix/ui.scm:2164:12: In procedure run-guix-command:
Bad Read-Header-Line header: #<eof>

substitution of /gnu/store/c39c4jhgg1pq3sw083pcmp42gba05kr2-evince-3.36.5 failed
```

I'm finding them all the time.

I think they are very cryptic... Maybe we can do something
about that.

Best,
Ekaitz