token_get_entity_mapping() should fallback to the entity type if token type not found

If we pass in an invalid entity type to token_get_entity_mapping('entity', $entity_type), I have a suspicion that the returned value should be the passed in invalid entity type rather than FALSE. It causes problems like #1773926: Token validaiton fails on config edit if the instance context isn't an entity type. Maybe we should add an *optional* fallback for when we don't care for a FALSE value or not.