Error if game already started or more than 4 players

public/index.html

@@ -24,6 +24,7 @@
 <main>
     <div id="login">
         <form>
+            <h2 id="error"></h2>
             <label for="username">Benutzername: </label>
             <input id="username" name="username" placeholder="Benutzername" type="text">