Add missing 422 status code in post/patch/delete relationship endpoint

[bkoelman]

Fixes #1700. Includes additional cleanup of tests.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.34%. Comparing base (5c2f2ec) to head (0a5d2c5).
Report is 5 commits behind head on openapi.

Additional details and impacted files

@@             Coverage Diff             @@
##           openapi    #1701      +/-   ##
===========================================
- Coverage    90.35%   90.34%   -0.02%     
===========================================
  Files          462      462              
  Lines        14076    14080       +4     
  Branches      2198     2198              
===========================================
+ Hits         12719    12720       +1     
- Misses         918      921       +3     
  Partials       439      439              

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