cmd/loop: set exit code 1 on error

cmd/loopd/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"os"
 
 	"github.com/lightninglabs/loop/loopd"
 )
@@ -11,5 +12,6 @@ func main() {
 	err := loopd.Run(cfg)
 	if err != nil {
 		fmt.Printf("loopd exited with an error: %v\n", err)
+		os.Exit(1)
 	}
 }