MR widget: Clarify ambiguous parsing failure message

What does this MR do and why?

Clarify ambiguous instructions, which led investigation for this internal request into a wrong direction initially. The code comment and the front-end message seemed contradictory to me, with the former being for more useful.

Follow-up to !136990 (diffs)

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Is this needed for a 1 line message change?

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Related to #431850 (closed)