controllers/krate/publish: Simplify "duplicate version" error code

[Turbo87]

This PR makes the error handling for the "version XXX already exists" case a little easier on the 👀

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.80%. Comparing base (3ce70e7) to head (7f0b098).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/controllers/krate/publish.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9774      +/-   ##
==========================================
- Coverage   88.80%   88.80%   -0.01%     
==========================================
  Files         289      289              
  Lines       29809    29808       -1     
==========================================
- Hits        26472    26471       -1     
  Misses       3337     3337              

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