Django helo me raise validation error

I am trying to create some kind of warning when someone enters the username that was already taken. But all i manage to do is to redirect to static page where it says: username picked OR passwords do not match..

this is what i have written in my views.py:

def register_user(request):
    if request.method == 'POST':
        form = MyRegistrationForm(request.POST)
        if form.is_valid():
            form.save()
return HttpResponseRedirect('/accounts/register_success')
        else:
            return render_to_response('invalid_reg.html')

    args = {}
    args.update(csrf(request))

    args['form'] = MyRegistrationForm()
    print args
    return render_to_response('register.html', args)

and this is my forms.py:

class MyRegistrationForm(UserCreationForm):
    email = forms.EmailField(required=True)

    class Meta:
        model = User
        fields = ('username', 'email', 'password1', 'password2')

    def save(self, commit=True):
        user = super(MyRegistrationForm, self).save(commit=False)
        user.email = self.cleaned_data['email']
        user.set_password(self.cleaned_data["password1"])

        if commit:
            user.save()

        return user

Please help me :( i have literraly no idea how do i give a proper warning for the user :((