chainreg: require FeeUrl when running neutrino mainnet

Fixes #4770.

chainreg/chainregistry.go

@@ -582,8 +582,18 @@ func NewChainControl(cfg *Config) (*ChainControl, error) {
 			homeChainConfig.Node)
 	}
 
+	switch {
+	// If the fee URL isn't set, and the user is running mainnet, then
+	// we'll return an error to instruct them to set a proper fee
+	// estimator.
+	case cfg.FeeURL == "" && cfg.Bitcoin.MainNet &&
+		homeChainConfig.Node == "neutrino":
+
+		return nil, fmt.Errorf("--feeurl parameter required when " +
+			"running neutrino on mainnet")
+
 	// Override default fee estimator if an external service is specified.
-	if cfg.FeeURL != "" {
+	case cfg.FeeURL != "":
 		// Do not cache fees on regtest to make it easier to execute
 		// manual or automated test cases.
 		cacheFees := !cfg.Bitcoin.RegTest