Can you share your code (or a reduced test case), and a screenshot of
the error you're getting? I just tried writing the following hook:
https://phabricator.wikimedia.org/P10258
And defined the error test in MediaWiki:Custom-test-error on my test wiki.
And the VisualEditor uploader displayed the expected error message:
https://phabricator.wikimedia.org/F31532153
I was testing on MediaWiki 1.35.0-alpha, so perhaps something changed
since 1.32.2, but please check if that method works for you.
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Bartosz Dziewoński