Fixed #21884 -- Mentioned that exact is the defaut QuerySet lookup type

Thanks to cjerdonek for the suggestion.
Fixed #21765 -- Added support for comparing Context instances
Fixed #21912 -- Improved error message if a view returns None.

Thanks brycenesbitt for the report.
Fixed some flake8 errors from recent commits.
Fixed #22047 -- detecting related_name clash with inheritance

Thanks to mondone for fruitful colaboration.
Improved fix for #18373 -- backward compatibility

Commit 79558c78 cleaned up the (undocumented) interface of Resolver404
exception, which breaks compatibility with code messing with .args[0]
directly. Revert the cleanup part and simply leave the fix itself.