How to check if username or email has been taken already? [Using Parse]

Hi everyone.
Sorry if my english is not good enough...
I'm developing a Login and SignUp display using Parse... this is my first time using something related to databases or servers.

I've done the sign up button works if the username and email aren't register in my parse plattform, and the loggin button works if the password and username exists in the plattform and they match. But if something is wrong whit it, How can I know if the username is registered or the password is wrong?
here is the documentation of Parse:
https://parse.com/docs/unity/guide#users

And here is my code for Login:

 public void LogIn()
     {
         ParseUser.LogInAsync (usuario.text, contrasenia.text).ContinueWith (t => 
         {
             if (t.IsFaulted || t.IsCanceled) {
                 //login Failed
                 print ("FAILED LOGIN");
             } 
             else {
                 //Login was Succesful
                 print ("LOGIN SUCCESS");
             }
         });}

And here is my code for SignUp:

 using UnityEngine;
 using System.Collections;
 using Parse;
 using System.Threading.Tasks;
 
 public class AddUserTEST : MonoBehaviour {
         
     private ParseUser user;
 
     void Start(){
 
 
     }
 
 void SuscribirUsuario(){
         user = new ParseUser ()
         {
             Username = "exampleUsername",
             Password = "examplePassword",
             Email = "mailExample@hotmail.com"
 
         };
 
         user["Nivel"] = "1";
 
         Task signUpTask = user.SignUpAsync () ;
 
 
     }
     void Update(){
         if(Input.GetKeyDown(KeyCode.Space)){
             SuscribirUsuario();
 
         }
 
     }
 
 
 
 
 
 
 
 
 }
 

Thanks everyone... :)