Improve product update form error handling

[dominik-zeglen]

Resolves #386

Pull Request Checklist

1. All visible strings are translated with proper context.
2. All data-formatting is locale-aware (dates, numbers, and so on).
3. Translated strings are extracted to .pot file.
4. Number of API calls is optimized.
5. The changes are tested.
6. Type definitions are up to date.
7. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #392 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #392   +/-   ##
=======================================
  Coverage   72.45%   72.46%           
=======================================
  Files         639      639           
  Lines        7116     7118    +2     
  Branches     1183     1184    +1     
=======================================
+ Hits         5156     5158    +2     
  Misses       1719     1719           
  Partials      241      241           

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