fix: postgresql migration not working with other languages

[fmartingr]

Fix an error for postgres migrations on different languages, checking for the error code from postgres instead of comparing strings.

Closes #996

[codecov]

Codecov Report

Attention: Patch coverage is 53.84615% with 12 lines in your changes missing coverage. Please review.

Project coverage is 34.72%. Comparing base (7b6fad8) to head (0c18db9).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/database/pg.go	53.84%	10 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1013      +/-   ##
==========================================
+ Coverage   34.66%   34.72%   +0.06%     
==========================================
  Files          61       61              
  Lines        5316     5324       +8     
==========================================
+ Hits         1843     1849       +6     
- Misses       3249     3251       +2     
  Partials      224      224              

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