'Dummy access token used when an invalid token was supplied.
:returns: The dummy access token string.
The dummy access token should be associated with an access token
secret such that get_access_token_secret(.., dummy_access_token)
returns a valid secret.
This method is used by
* ResourceEndpoint'

@property
def dummy_access_token(self):

    raise NotImplementedError(u'Subclasses    must    implement    this    function.')
