rebuilderd_stats: Add category for network errors

Example from <https://amd64.reproduce.debian.net/api/v0/builds/143479/log>:

```
+ dirname /tmp/rebuilderdEtJqAh/inputs/raincat_1.2.3-1_amd64.buildinfo
+ cd /tmp/rebuilderdEtJqAh/inputs
+ mkdir -p etc/apt
+ mkdir -p var/lib/apt/lists/
+ echo deb-src [signed-by=/usr/share/keyrings/debian-archive-keyring.gpg] https://deb.debian.org/debian trixie main
+ apt-get -o Dir=. update
Ign:1 https://deb.debian.org/debian trixie InRelease
Ign:1 https://deb.debian.org/debian trixie InRelease
Ign:1 https://deb.debian.org/debian trixie InRelease
Err:1 https://deb.debian.org/debian trixie InRelease
  Could not connect to 127.0.0.1:3128 (127.0.0.1). - connect (111: Connection refused)
  Unable to connect to 127.0.0.1:3128:
Reading package lists...
W: Failed to fetch https://deb.debian.org/debian/dists/trixie/InRelease

  Unable to connect to 127.0.0.1:3128:
W: Some index files failed to download. They have been ignored, or old ones used instead.
+ basename /tmp/rebuilderdEtJqAh/inputs/raincat_1.2.3-1_amd64.buildinfo
+ cut -d_ -f1
+ apt-get -o Dir=. source --download-only raincat
Reading package lists...
E: You must put some 'deb-src' URIs in your sources.list
```

Signed-off-by: Holger Levsen <holger@layer-acht.org>