...
user = authenticate(username=username, password=password)
if user:
    if user.is_active:
        login(request, user)
        return redirect('index')
    else:
        message = "Your account is disabled."
else:
    message = "Invalid username or password supplied."
...

if user:
...
else:
    if (LDAP auth failed reason == user does not satisfy AUTH_LDAP_REQUIRE_GROUP):
       message = "You are not in the right user group."
    else:
       message = "Invalid username or password supplied."
...