Improve exception message for invalid filter result

[rpkilby]

As mentioned in #938, the exception for an invalid filter result isn't that helpful. e.g., if a custom filter method unintentionally returns a None value, you end up with NoneType has no attribute filter, which lacks any context about the offending FilterSet and Filter instance.

@jpic - would this exception have been helpful the first time around? e.g.

Expected 'F.invalid' to return a QuerySet, but got a NoneType instead.

[codecov-io]

Codecov Report

Merging #943 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #943      +/-   ##
==========================================
+ Coverage   97.95%   98.04%   +0.09%     
==========================================
  Files          15       15              
  Lines        1122     1123       +1     
==========================================
+ Hits         1099     1101       +2     
+ Misses         23       22       -1

Impacted Files	Coverage Δ
django_filters/filterset.py	100% <100%> (ø)	⬆️
django_filters/filters.py	98.74% <0%> (+0.31%)	⬆️

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[jpic]

LGTM

[stefanw]

I would like to use django-filter for things besides Django's querysets like Haystack's SearchQuerySet. The assertion in this PR with an isinstance (not even issubclass) is a bit too assuming. Maybe a None check would suffice to report a common error?

[rpkilby]

Hi @stefanw. django-filter isn't going to be compatible with SearchQuerySet since a SQS is not compatible with the standard QuerySet API. They have similarities, but they are not compatible with each other. e.g., you cannot combine a SearchQuerySet with a standard QuerySet via & or |, SearchQuerySet cannot accept Q objects, etc...

The filter_queryset method is functionally 3 lines of code. If you want django-haystack compatibility, I would recommend creating a custom base filterset class that overrides this method.