CW2: add a cw2::VersionError error type

[larry0x]

A major use case of cw2 is when migrating a contract, check the stored version info to make sure we're not migrating a wrong contract, or migrating from a wrong version. Therefore, it makes sense to add a VersionError type which is to be emitted if this check fails:

#[derive(thiserror::Error, Debug)]
enum VersionError {
    #[error("contract version info not found")]
    NotFound,

    #[error("wrong contract: expecting `{expected}`, found `{found}`")]
    WrongContract {
        expected: String,
        found: String,
    },

    #[error("wrong contract version: expecting `{expected}`, found `{found}`")]
    WrongVersion {
        expected: String,
        found: String,
    },
}

// run this as part of the migration process
fn assert_version(
    storage: &dyn Storage,
    expected_contract: &str,
    expected_version: &str,
) -> Result<(), VersionError> {
    let Some(ContractVersion { contract, version }) = CONTRACT.may_load(storage)? else {
        return Err(VersionError::NotFound);
    };

    if contract != expected_contract {
        return Err(VersionError::WrongContract {
            expected: expected_contract.into(),
            found: contract,
        });
    }

    if version != expected_version {
        return Err(VersionError::WrongVersion {
            expected: expected_version.into(),
            found: version,
        });
    }

    Ok(())
}