[core] 500 error page is dependent monkey patched method

class DesktopBackendBase(object):
  """
  Abstract base class for providing external authentication schemes.

  Extend this class and implement check_auth
  """
  def authenticate(self, username, password):
    if self.check_auth(username, password):
      user = find_or_create_user(username)
      user = rewrite_user(user)
      return user
    else:
      return None

  def get_user(self, user_id):
    try:
      user = User.objects.get(pk=user_id)
      user = rewrite_user(user)
      return user
    except User.DoesNotExist:
      return None

  def check_auth(self, username, password):
    """
    Implementors should return a boolean value which determines
    whether the given username and password pair is valid.
    """
    raise Exception("Abstract class - must implement check_auth")