small improvements

- renamed _Error -> BaseError
- comparing BaseErrors checks `obj` fields
- rewritten some CharFieldTests -- added `obj` field to Error instances
rewritten checks of Field.choices
rewritten checks of Field.db_index
rewritten checks of DecimalField
rewritten checks of FileField.upload_to
rewritten checks of BooleanField.null
rewritten checks of GenericIPAddressField
small fixup -- rewritting checks of CharFields