More descriptive error message when invalid slot duration is used (#6430)

* Initial commit

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* Errors if slot_duration is zero

* Errors if slot_duration is zero

* Revert "Errors if slot_duration is zero"

This reverts commit a9e9820e124571f73d3e498e969a74d01fd3fe96.

* Update client/consensus/slots/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>