CronCat errors macro

This package creates a procedural attribute macro that creates #[croncat_error], intended to be placed above the integrating contract's error enum, which is conventionally called ContractError.

Note: this macro should go above the usual Derive we see for this enum. The correct placement looks like this:

```rs

[croncat_error]

[derive(Error, Debug, PartialEq)]

pub enum ContractError { // variants here… } ```

It will throw a compile-time error if it is not placed correctly.

The macro adds this enum variant:

```rs

[error("CronCat error: {err:?}")]

CronCatError { err: CronCatContractError } ```

It also adds this logic:

rs impl From<CronCatContractError> for ContractError { fn from(error: CronCatContractError) -> Self { ContractError::CronCatError { err: error, } } }

The above logic will allow helper functions in croncat-integration-utils to propagate errors in a manner consistent with the contract's errors.