changed error message when ImageField is in use and Pillow is not installed
small cleanup: indented under-indented line
'swappable user model' -> 'custom user model'
'pointed' -> 'pointed to'
Warning -> Error for check if username is not included in REQUIRED_FIELDS
changed the way requires_model_validation/requires_checks options are handled
imported FieldDoesNotExist directly in _check_ordering