Packet worker/controller: improve wait-for-dns output

[margamanterola]

dig returns 0 even if the domain passed doesn't exist. So the script as is was printing Failed with exit code 0: when there was a nameserver configuration error, which was not a useful error message:

● wait-for-dns.service - Wait for DNS entries
   Loaded: loaded (/etc/systemd/system/wait-for-dns.service; enabled; vendor preset: enabled)
   Active: activating (start) since Mon 2020-07-27 14:20:17 UTC; 9min ago
 Main PID: 7128 (sh)
    Tasks: 3 (limit: 32767)
   Memory: 5.3M
   CGroup: /system.slice/wait-for-dns.service
           ├─ 7128 /bin/sh -c while ! /usr/bin/grep ^[^#[:space:]] /etc/resolv.conf > /dev/null; do sleep 1; done; /o>
           ├─ 7130 /bin/bash /opt/wait-for-dns infra.flatcar-linux.net monitoring-private 3600
           └─17621 sleep 1

Jul 27 14:29:39 monitoring-controller-0 sh[7128]: Failed with exit code 0:
Jul 27 14:29:40 monitoring-controller-0 sh[7128]: Failed with exit code 0:
Jul 27 14:29:42 monitoring-controller-0 sh[7128]: Failed with exit code 0:
Jul 27 14:29:44 monitoring-controller-0 sh[7128]: Failed with exit code 0:
Jul 27 14:29:46 monitoring-controller-0 sh[7128]: Failed with exit code 0:
Jul 27 14:29:48 monitoring-controller-0 sh[7128]: Failed with exit code 0:
Jul 27 14:29:49 monitoring-controller-0 sh[7128]: Failed with exit code 0:
Jul 27 14:29:51 monitoring-controller-0 sh[7128]: Failed with exit code 0:
Jul 27 14:29:53 monitoring-controller-0 sh[7128]: Failed with exit code 0:
Jul 27 14:29:55 monitoring-controller-0 sh[7128]: Failed with exit code 0:

Instead, modify these scripts to print the actual error (that the nameservers weren't found) when the output is empty.

[invidian]

Thanks for the PR @marga-kinvolk. Please run make update-assets and commit generated code together with your changes, to make the CI pass.

I also pointed one suggestion in the comment.

[ipochi]

Thanks for the PR @marga-kinvolk

+1 for make update-assets , even running make should suffice. Generated files is pkg/assets/generated_assets.go

[margamanterola]

Sorry, I missed the callout to make update-assets before. Done now.

[invidian]

LGTM

[ipochi]

LGTM.