Add support for validation error codes

[rpkilby]

This is for both the base FilterSet and the DRF variant.

• The base FilterSet is slightly broken, since it simply raises ValidationError(self.form.errors). This loses the individual validation error codes.
• The raw_validation approach needs to be modified to return an actual serializers.ValidationError, since the plain data structure lacks the error codes.

(This is WIP)

[codecov-io]

Codecov Report

Merging #702 into develop will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           develop     #702   +/-   ##
========================================
  Coverage    98.13%   98.13%           
========================================
  Files           15       15           
  Lines         1123     1123           
========================================
  Hits          1102     1102           
  Misses          21       21

Impacted Files	Coverage Δ
django_filters/filterset.py	100% <100%> (ø)	⬆️

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[rpkilby]

I'm currently stuck on this. I'm 95% sure there's a bug preventing DRF ValidationErrors from having custom error codes. In essence, there's this section which interacts with how ValidationErrors coerce their string messages to a list here. In short, instead of getting an ErrorDetail with a string and code, you just get a list of strings.

I need to look into this further.

[rpkilby]

Note to self... https://github.com/encode/django-rest-framework/issues/4787#issuecomment-326062971

[blueyed]

You might want to keep codes here:

        error_dict = OrderedDict([
            (key, [exceptions.ErrorDetail(e.message,
                                          e.code if e.code else default_code)
                   for e in error_list])
            for key, error_list in error_dict.items()])
        drf_validation_error = exceptions.ValidationError(error_dict)

[blueyed]

See encode/django-rest-framework#4787 (comment) for an updated version that also handles params.

[rpkilby]

I'm closing this off in favor of #788, which is generally reworking filterset validation.