Clarify error message

[schmidt-sebastian]

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[changeset-bot]

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Latest commit: bdfd390

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[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (762fa53) and head commit (a5984c4).

Test Logs

• Base (762fa539): https://github.com/firebase/firebase-js-sdk/actions/runs/483347953
• Head (a5984c4a): https://github.com/firebase/firebase-js-sdk/actions/runs/483470994