Have kv-store#open return an optional handle

[elliottt]

Modify the definition of the open function in the kv-store interface to return an optional handle instead of wrapping that case into the error.

[elliottt]

There are various functions where the optional-none error is used. Perhaps this error code can be entirely deprecated in the WIT.

Yep, I'd like to deprecate that error value, but haven't audited all the calls yet to make sure they're all migrated.