standard error messages in pbori

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no capital, no final dot

Component: **commutative algebra**

Keywords: **pbori**

Author: **Frédéric Chapoton**

Branch/Commit: **[`82bfeaa`](https://github.com/sagemath/sagetrac-mirror/commit/82bfeaaaf6b772d0c893b12e7945707c745f0c0e)**

Reviewer: **David Coudert**

_Issue created by migration from https://trac.sagemath.org/ticket/34578_

