Returns the provided token or the default token.

  Args:
    token: A token or None.

  Raises:
    access_control.UnauthorizedAccess: no token was provided.

def GetDefaultToken(token):
  """Returns the provided token or the default token.

  Args:
    token: A token or None.

  Raises:
    access_control.UnauthorizedAccess: no token was provided.
  """
  if token is None:
    token = default_token

  if not isinstance(token, access_control.ACLToken):
    raise access_control.UnauthorizedAccess(
        "Token is not properly specified. It should be an "
        "instance of grr.lib.access_control.ACLToken()")

  return token