feat: do no exit with error when config file is not set

If the default config file (config.yml) doesn't exist, don't exit with
an error. Instead, use a default configuration file. This removes a
small point of friction when gettting started with go-librespot.

On the other hand, if the config path is set explicitly using
`-config_path`, do exit with an error when it cannot be found (because
presumably the user made a mistake there).

cmd/daemon/main.go

@@ -354,13 +354,19 @@ type Config struct {
 }
 
 func loadConfig(cfg *Config) error {
-	flag.StringVar(&cfg.ConfigPath, "config_path", "config.yml", "the configuration file path")
+	flag.StringVar(&cfg.ConfigPath, "config_path", "", "the configuration file path (default config.yml)")
 	flag.StringVar(&cfg.CredentialsPath, "credentials_path", "credentials.json", "the credentials file path")
 	flag.Parse()
 
-	configBytes, err := os.ReadFile(cfg.ConfigPath)
+	configPath := cfg.ConfigPath
+	if configPath == "" {
+		configPath = "config.yml"
+	}
+	configBytes, err := os.ReadFile(configPath)
 	if err != nil {
-		return fmt.Errorf("failed reading configuration file: %w", err)
+		if cfg.ConfigPath != "" || !os.IsNotExist(err) {
+			return fmt.Errorf("failed reading configuration file: %w", err)
+		}
 	}
 
 	if err := yaml.Unmarshal(configBytes, cfg); err != nil {