Add handling of nested errors

[sliverc]

Fixes #438
Fixes #416

Description of the Change

Properly handle nested errors. Especially important for nested serializer structure introduced in #776

Extracted from PR #787

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #815 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #815      +/-   ##
==========================================
+ Coverage   97.45%   97.52%   +0.07%     
==========================================
  Files          56       58       +2     
  Lines        3068     3157      +89     
==========================================
+ Hits         2990     3079      +89     
  Misses         78       78              

Impacted Files	Coverage Δ
example/tests/test_generic_viewset.py	100.00% <ø> (ø)
example/tests/snapshots/snap_test_errors.py	100.00% <100.00%> (ø)
example/tests/test_errors.py	100.00% <100.00%> (ø)

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[n2ygk]

I'm looking forward to these improved error messages. Thanks for this.