Assets: can_decrease/increase for destroying asset is not successful (#3286)

Functions `can_decrease` and `can_increase` do not return successful
consequence results for assets undergoing destruction; instead, they
return the `UnknownAsset` consequence variant.

This update aligns their behavior with similar functions, such as
`reducible_balance`, `increase_balance`, `decrease_balance`, and `burn`,
which return an `AssetNotLive` error for assets in the process of being
destroyed.