Add code to error object if ErrorDetail has code

[josebama]

Description of the Change

JSON:API specification allows for a code value in the error object. This PR copies the code inside the ErrorDetail from rest_framework's APIException into the JSON:API error object. Allowing users of the library to send application-specific error codes back to the clients by specifying a code alongside the detail when raising an APIException

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 #700 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #700   +/-   ##
=======================================
  Coverage   95.97%   95.97%           
=======================================
  Files          54       54           
  Lines        2735     2735           
=======================================
  Hits         2625     2625           
  Misses        110      110

Impacted Files	Coverage Δ
example/tests/test_generic_validation.py	100% <ø> (ø)	⬆️
example/tests/test_generic_viewset.py	100% <ø> (ø)	⬆️
example/tests/test_model_viewsets.py	100% <ø> (ø)	⬆️
example/tests/test_views.py	100% <ø> (ø)	⬆️

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[codecov]

Codecov Report

Merging #700 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #700   +/-   ##
=======================================
  Coverage   95.97%   95.97%           
=======================================
  Files          54       54           
  Lines        2735     2735           
=======================================
  Hits         2625     2625           
  Misses        110      110

Impacted Files	Coverage Δ
example/tests/test_generic_validation.py	100% <ø> (ø)	⬆️
example/tests/test_generic_viewset.py	100% <ø> (ø)	⬆️
example/tests/test_model_viewsets.py	100% <ø> (ø)	⬆️
example/tests/test_views.py	100% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 252d164...d2c8e47. Read the comment docs.

[sliverc]

Nice work and a great improvement! Thanks.