Solved by XRENDERING-621 in 14.0RC1: macro errors are no longer hardcoded in MacroErrorManager. They are rendered by an overridable ErrorBlockGenerator component, and in xwiki-platform by XWikiErrorBlockGenerator, which displays each error through the template xwikirenderingerror/<messageId>.vm, falling back on xwikirenderingerror/default.vm. A wiki can customize the general error message by overriding these templates in its skin, which is what this issue asked for.
Making the cause of a macro execution failure translatable is still open in XRENDERING-720.
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