better error message in magma_free when timeout

[fchapoton]

fix #38430

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[maxale]

@fchapoton The fix looks good to me. However, in addition to fixing the error message, it should be also mentioned in the documentation for magma_free that in the case of timeout the TimeoutError exception is raised. Currently, it just says:

The code must evaluate in at most 120 seconds and there is a limitation on the amount of RAM.

without saying what would happen when time is out.