Return an error message

You can return a specific error message from your contract via ink::return_value(REVERT, err);.

If you do a dry-run of the contract call, you can read this data.

You cannot read the data if you submit the extrinsic on-chain (i.e. execute the contract call "for real", on-chain). This is because extrinsic do not return data. They are executed in a transaction pool, asynchronous.

Furthermore, since you "REVERT" the transaction, it will just fail with ExtrinsicFailed.

Here's an example:

#[ink::message]
fn get(&self) {
    ink::return_value(
        ink::env::ReturnFlags::REVERT,
        format!("received {:?}", self.env().transferred_value()).as_bytes();
    );
}

We've put the above into a complete example. You can see the full source code here.