Raise specific DoesNotExist exception for the model.

[donkirkby]

Also add some missing comparisons.

Two small features we needed for our tests.

[codecov-io]

Codecov Report

Merging #32 into master will not impact coverage.

@@          Coverage Diff          @@
##           master    #32   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           6      6           
  Lines         479    486    +7     
=====================================
+ Hits          479    486    +7

Impacted Files	Coverage Δ
django_mock_queries/utils.py	100% <ø> (ø)	✅
django_mock_queries/constants.py	100% <100%> (ø)	✅
django_mock_queries/query.py	100% <100%> (ø)	✅

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[stphivos]

Thanks!!