improve error message when trying to cancel before timelock 
after locking btc, attempting to cancel before the timelock gives this error 

```
Error: Failed to broadcast Bitcoin cancel transaction ...

Caused by:
    Electrum(Protocol(String("sendrawtransaction RPC error: {\"code\":-26,\"message\":\"non-BIP68-final\"}")))
    ``` 

This should be a better error message indicating the status 