fix(user-create): duplicate email error message

[gagantrivedi]

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Changes

Return a generic error message for user create if the email is not unique

How did you test this code?

covered by unit test cases and verified manually by creating a user with duplicate email

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[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (1514bf7) 95.44% compared to head (ecfb3eb) 95.43%.
Report is 16 commits behind head on main.

❗ Current head ecfb3eb differs from pull request most recent head f34b363. Consider uploading reports for the commit f34b363 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2642      +/-   ##
==========================================
- Coverage   95.44%   95.43%   -0.01%     
==========================================
  Files         976      975       -1     
  Lines       27319    27253      -66     
==========================================
- Hits        26074    26010      -64     
+ Misses       1245     1243       -2     

Files Changed	Coverage Δ
api/audit/serializers.py	100.00% <100.00%> (ø)
api/custom_auth/serializers.py	100.00% <100.00%> (ø)
api/custom_auth/tests/test_serializer.py	100.00% <100.00%> (ø)
api/environments/serializers.py	95.52% <100.00%> (-0.32%)	⬇️
api/environments/views.py	90.59% <100.00%> (+1.16%)	⬆️
api/organisations/views.py	91.16% <100.00%> (ø)
api/projects/serializers.py	100.00% <100.00%> (ø)
api/projects/tests/test_serializers.py	100.00% <100.00%> (ø)
api/projects/views.py	94.01% <100.00%> (-0.30%)	⬇️

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