title: Django how to change the name of Custom User Model in error messages in a form
body: <p>I have created a sign up form called <code>UserRegistrationForm</code> using <code>UserCreationForm</code> in django and have a custom user model name <code>CustomUserModel</code>.</p>
  <p>When I sign up using wrong credentials, the error messages show &quot;Custom user model with this User name already exists.&quot;. I would like it to be something like &quot;Username already exists&quot;. How can I do that?</p>
  <p><strong>Forms.py</strong></p>
  <pre class="lang-py prettyprint-override"><code>class UserRegistrationForm(UserCreationForm):
      user_name = forms.CharField(max_length=150)
      email = forms.EmailField()
      first_name = forms.CharField(max_length=120)
      last_name = forms.CharField(max_length=120, required=False)
      date_of_birth = forms.DateField(
          widget=forms.DateInput(
              format='%d/%m/%Y',
              attrs={'type': 'date', 'min': '1900-01-01', 'max': str(timezone.now().year)+'-'+str(timezone.now().month)+'-'+str(timezone.now().day)},
          )
      )
      gender = forms.ChoiceField(
          widget=forms.Select, 
          choices=(('M', 'Male'), ('F', 'Female'), ('T', 'Transgender'), ('N', 'NonBinary')), 
          initial='N',
      )
  
      def is_valid(self):
          form = super(UserRegistrationForm, self).is_valid()
          print(&quot;*******************************&quot;)
          print(self.errors)
          print(&quot;*******************************&quot;)
          return form
  </code></pre>
  <p><strong>CustomUserModel</strong></p>
  <pre><code>class CustomUserModel(AbstractBaseUser, PermissionsMixin):
      email = models.EmailField(_('email address'), unique=True)
      user_name = models.CharField(max_length=150, unique=True)
      first_name = models.CharField(max_length=120)
      last_name = models.CharField(max_length=120, blank=True)
      date_of_birth = models.DateField(max_length=8)
      date_created = models.DateTimeField(default=timezone.now) 
      gender = models.CharField(max_length=1, default='N', choices=(('M', 'Male'), ('F', 'Female'), ('T', 'Transgender'), ('N', 'NonBinary')))
      is_staff = models.BooleanField(default=False)
      is_active = models.BooleanField(default=True)
  
      objects = CustomAccountsManager()
  
      USERNAME_FIELD = 'user_name'
      REQUIRED_FIELDS = ['email', 'first_name', 'date_of_birth']
  
      def __str__(self):
          return self.user_name
  </code></pre>
  <p>Here is a picture to help you understand my problem.</p>
  <p><a href="https://i.stack.imgur.com/ilycy.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ilycy.jpg" alt="Illustration of form" /></a></p>
  <p>The error states &quot;Custom user model with this User name already exists.&quot; I want to change this message</p>