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 a9e9820e

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

Co-authored-by: Bastian Köcher <[email protected]>