cmd/node: recover setting conf.Vochain.Network

An earlier commit removed that line as I thought it was unnecessary.
I missed that "chain" isn't bound to Vochain.Network,
because the flag name isn't in the "vochainNetwork" format to match.

Add it back with enough docs so that we know why it's needed,
and improve the error so it isn't confusing on empty strings:

    cannot find a valid genesis for the  network

cmd/node/main.go

@@ -267,8 +267,10 @@ func loadConfig() *config.Config {
 	if err != nil {
 		log.Fatalf("cannot unmarshal loaded config file: %s", err)
 	}
-	// Note that conf.Vochain.Indexer isn't bound with viper.
+	// Note that these Config.Vochain fields aren't bound via viper.
+	// We could do that if we rename the flags, e.g. vochainIndexerArchiveURL.
 	conf.Vochain.Indexer.ArchiveURL = viper.GetString("archiveURL")
+	conf.Vochain.Network = viper.GetString("chain")
 
 	if conf.SigningKey == "" {
 		log.Info("no signing key, generating one...")

service/vochain.go

@@ -74,7 +74,7 @@ func (vs *VocdoniService) Vochain() error {
 						return err
 					}
 					if _, ok := vocdoniGenesis.Genesis[vs.Config.Network]; !ok {
-						err = fmt.Errorf("cannot find a valid genesis for the %s network", vs.Config.Network)
+						err = fmt.Errorf("cannot find a valid genesis for the %q network", vs.Config.Network)
 						return err
 					}
 					genesisBytes = vocdoniGenesis.Genesis[vs.Config.Network].Genesis.Marshal()