title: How can I change the "account already exists" message in Django allauth?
body: <p>When trying to log in with a social account while there already being an account with that email, the following message shows:</p>
  
  <p><code>An account already exists with this e-mail address. Please sign in to that account first, then connect your Google account.</code></p>
  
  <p>Now I would like to change that message. At first I tried to override <code>ACCOUNT_SIGNUP_FORM_CLASS = 'mymodule.forms.MySignupForm'</code> and gave it my own <code>raise_duplicate_email_error</code> method, but that method is never called.</p>
  
  <p>The form looks like this:</p>
  
  <pre><code>class SignupForm(forms.Form):
      first_name = forms.CharField()
      last_name = forms.CharField()
      boolflag = forms.BooleanField()
  
      def raise_duplicate_email_error(self):
          # here I tried to override the method, but it is not called
          raise forms.ValidationError(
              _("An account already exists with this e-mail address."
                " Please sign in to that account."))
  
      def signup(self, request, user):
          # do stuff to the user and save it 
  </code></pre>
  
  <p>So the question is: How can I change that message?</p>