Check if url flag is valid

main.go

@@ -3,12 +3,13 @@ package main
 import (
 	"encoding/json"
 	"flag"
+	"fmt"
 	"io"
 	"log"
 	"net/http"
 	"os"
-	"strings"
 	"strconv"
+	"strings"
 )
 
 func main ()  {
@@ -23,6 +24,13 @@ func main ()  {
 	
 	flag.Parse()
 	
+	// Check if flags are valid
+	if *url == "" {
+		fmt.Println("no thread URL provided")
+		fmt.Println("use the -u=<Thread URL> flag to provid URL")
+		os.Exit(1)
+	}
+
 	// Getting the boardname
 	board := strings.Split(*url, "/")[3]