gh-41041: better error message in magma_free when timeout
fix #38430
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
URL: https://github.com/sagemath/sage/pull/41041
Reported by: Frédéric Chapoton
Reviewer(s):
