Add optional kwarg to serializer.is_valid method for passing custom api exception class

[serotonin276]

Currently we have same code value for all the validation errors, which is invalid. There is no way to pass in custom value, because the is_valid method of serializer internally raises the ValidationError exception. The method only passes detail after collecting all the validation error details. It would be nice if the method had a optional code kwarg defaulting to NoneSimilarly, there is no way, if we want to categorize and add more details to the ValidationError class.

Or add anSo I suggest to add an optional kwarg for passing a custom api exception class defaulting to the ValidationError class, so that users can construct their own error response for validation errors