silkworm: unexpected termination when STUN server cannot be resolved

Running Silkworm on some machine behind strict firewall rules may lead to this unexpected termination to happen if Google STUN server (stun.l.google.com) default port (19302) is not open:
```
INFO [12-23|18:32:57.658 UTC] Silkworm version=master0.1.0-dev+commit.b0ce84b9 build=linux-x86_64 release compiler=gnu 11.4.0
...
INFO [12-23|18:32:57.789 UTC] Silkworm is now running
INFO [12-23|18:32:57.789 UTC] SnapshotSync: downloading missing snapshots if needed
INFO [12-23|18:32:57.789 UTC] SnapshotSync: download started: [0/462]
CRIT [12-23|18:33:03.511 UTC] Unrecoverable failure: stun_ip_resolver: no response from the STUN server
```

We could try to detect such condition as accurately as possible and suggest to enable the command-line option `--nat extip:`.