Fix to return correct gRPC errors when creating duplicate relations

[uatuko]

Return ALREADY_EXISTS gRPC status when creating duplicate relations.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.51%. Comparing base (d00df95) to head (280bf6c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #120      +/-   ##
==========================================
+ Coverage   95.50%   95.51%   +0.01%     
==========================================
  Files          19       19              
  Lines        1246     1249       +3     
  Branches      141      141              
==========================================
+ Hits         1190     1193       +3     
  Misses         38       38              
  Partials       18       18              

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