fix(pallet-contracts): Add a handler for MemoryGrow.

[magistau]

This error kind was introduced with bb32803 (see #1), but pallet-contracts was not updated after adding it.

Note that adding a variant to an public exhaustive enum is a breaking change.
See https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new.

[ukint-vs]

This was done intentionally to have as minimal changes to upstream as possible since we are not using contracts pallet and we need to rebase each time. But we can just put unreachable maybe in grow fn.

[magistau]

Well, pallet-contracts doesn't compile at all without handling MemoryGrow and this change is simple enough not to cause complex conflicts