some minor details in misc

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in particular about error messages

Component: **misc**

Author: **Frédéric Chapoton**

Branch/Commit: **[`4402561`](https://github.com/sagemath/sagetrac-mirror/commit/44025613a6194e6a491f550c2dd91f2a4e7d8033)**

Reviewer: **Kwankyu Lee**

_Issue created by migration from https://trac.sagemath.org/ticket/34544_

