N2c local submission mini-protocol rejection structured reason

When submitting a message on the n2c local submission mini-protocol, if an error occurs (KES signature is invalid, already expired message, ...) it is reported to the client. Today, the error is a type and optionally an error message in text. We would like to create more structured optional explanation of the errors. This would require modifications of the CIP, Pallas library, Mithril nodes.

Current CDDL:
```cddl
reason = invalid
/ alreadyReceived
/ expired
/ other

invalid = [0, tstr]
alreadyReceived = [1]
expired = [2]
other = [3, tstr]
```