checkers: add unassignedError

[cristaloleg]

No description provided.

[un000]

IP validation failed

	if net.ParseIP(ipAddr).To4() == nil {
		continue
	}

Workers loop failed:

func runWorker(ctx context.Context) {
	for {
		if ctx.Err() != nil {
			return
		}
		// ...
	}
}

[cristaloleg]

As for me 2nd fail is pretty acceptable, regarding 1st - not sure. However making explicit error handling ('hey look, I'm checking error here, not something else') is more clear. WDYT ?

[quasilyte]

This needs to be rebased and re-generated.
After that, we can merge it.

[quasilyte]

Now we need to check the warnings for the external testdata.
If they're expected, they need to be added to the golden files (PR to the external testdata repo).