make error no display error more clear

main.go

@@ -39,6 +39,10 @@ func main() {
 		display = *overrideDisplay
 	}
 
+	if display == "" {
+		log.Fatalf("No DISPLAY environment variable set and no override provided. Please set DISPLAY or use --display.")
+	}
+
 	connType, target := resolveDisplay(display)
 	fmt.Printf("Proxying to %s (%s)\n", target, connTypeString(connType))