Reject missing explicit configuration files

config/user_config.go

@@ -356,6 +356,20 @@ func (c *Config) UpdateConfig(u *gumble.User) {
 	j.LocallyMuted = u.LocallyMuted() // Save LocallyMuted state to config
 }
 
+// RequireConfigFile verifies that an explicitly requested configuration file
+// exists and is a regular file. The default configuration remains optional.
+func RequireConfigFile(fn string) error {
+	path := resolvePath(fn)
+	info, err := os.Stat(path)
+	if err != nil {
+		return fmt.Errorf("config file %q: %w", path, err)
+	}
+	if info.IsDir() {
+		return fmt.Errorf("config file %q is a directory", path)
+	}
+	return nil
+}
+
 func NewConfig(fn *string) *Config {
 	var c *Config
 	c = &Config{}