Fix: replace numeric error codes with http constants

[steindvart]

Small fix of 'magic numbers' in code.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.37%. Comparing base (1efca4f) to head (925f2d9).

Files with missing lines	Patch %	Lines
raw_request.go	16.66%	5 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #227      +/-   ##
==========================================
+ Coverage   45.30%   46.37%   +1.06%     
==========================================
  Files          29       29              
  Lines        2355     2098     -257     
==========================================
- Hits         1067      973      -94     
+ Misses       1241     1075     -166     
- Partials       47       50       +3     

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