This PR fixes the `0002-validators-warp-sync` zombienet test by matching
for a specific error case.

The following PR introduced a new error for validators that don't have
public addresses available for publishing in the DHT:
- https://github.com/paritytech/polkadot-sdk/pull/6298
The log line was moved from `eprintln!("Warning: ...` in sc-cli to the
authority discovery where it is printed properly as an error.


cc @paritytech/sdk-node

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>