Replace error with a warning when iptables fails

[LimeHat]

Fix #797

[LimeHat]

Makes sense @hellt, updated according to the comments

[hellt]

looks good @LimeHat

I had 3 messages around it this morning :D all the cases had one thing in common:
docker was installed via yum on RH 7.9 -> no DOCKER, DOCKER-USER chain whatsoever...

Initially I thought to make another check if DOCKER-USER chain exists and only then attempt to manipulate the rules. This will definitely mask any errors on installations which miss DOCKER-USER chain. But then maybe it is best to keep these errors present just to make people consider installing docker properly, so that chains will be there?
What do you think?

[LimeHat]

Hmm. Maybe add a check on the existence and a separate log message (perhaps with a link to a FAQ on the issue)?

[hellt]

sounds good

how about that?

kept under external-access chapter of network.md?

Source:

???error "I receive 'Missing Docker iptables chains' error"
    Containerlab will throw an error "Missing Docker iptables chains" when no `DOCKER-USER` chain is found. This source of this error is likely rooted in the way docker was installed on your system. Quite often, it happens when docker is installed via package managers.

    It is recommended to follow the [official installation procedures](https://docs.docker.com/engine/install/) by selecting "Installation per distro" menu option. When docker is correctly installed on the system, additional iptables chains will be available and the containerlab error will disappear.

If you want to finish this off you can take this and change the wording as you see fit (along with the additional check in the install/destroy funcs). Otherwise I can finish this off later