Make 404 & 403 responses consistent with exceptions.APIException output

[fengsi]

Map Django's 404 & 403 to DRF's exceptions, making responses consistent (otherwise default_code / code is not honored).

[rpkilby]

Going through the git blame, it looks like the current implementation is just an artifact of history. I don't know if there's a reason to distinguish Django's Http404 from DRF's NotFound. cc @tomchristie?

[tomchristie]

making responses consistent

I didn't immediately see any behavioral change here, but on a second pass I think I understand.
I assume you're globally setting the class attribute on one or both of NotFound/PermissionDenied someplace, eg...

NotFound. default_detail = '...'

I've got slightly mixed feelings about the change. I don't especially like changing exc rather than treating extra cases explicitly. However it is a little shorter, so probably okay with it.

[fengsi]

I had a custom exception handler which renames detail to error, and adds an error_code field as well:

def custom_exception_handler(exc, context):
    if isinstance(exc, Http404):
        exc = NotFound()
    elif isinstance(exc, Http403):
        exc = PermissionDenied()
    response = exception_handler(exc=exc, context=context)
    if response and isinstance(response.data, dict):
        detail = response.data.pop('detail', None)
        if detail:
            response.data['error'] = detail
        if hasattr(exc, 'get_codes'):
            response.data['error_code'] = exc.get_codes()
    return response

404 & 403 were hard-coded responses w/o the error_code part. This way they are both consistent with other APIException instances. :)

[blueyed]

Nice.
Reminded me of #5379 - this approach seems to be better.