Warn instead of crash when extract_feerate(...) fails

Fixes: #722

lightningd/bitcoind.c

@@ -255,10 +255,13 @@ static bool extract_feerate(struct bitcoin_cli *bcli,
 		      bcli_args(bcli),
 		      valid ? "partial" : "invalid");
 
-	if (tokens[0].type != JSMN_OBJECT)
+	if (tokens[0].type != JSMN_OBJECT) {
-		fatal("%s: gave non-object (%.*s)?",
+		log_unusual(bcli->bitcoind->log,
+			    "%s: gave non-object (%.*s)?",
 			    bcli_args(bcli),
 			    (int)output_bytes, output);
+		return false;
+	}
 
 	feeratetok = json_get_member(output, tokens, "feerate");
 	if (!feeratetok)