Harmonize log levels and error messages in `lightning-liquidity`

The logged and returned error messages are not super uniform across LSPS0/LSPS1/LSPS2 implementations. We should harmonize them and make sure they follow a somewhat uniform API language. 

As part of this we need to re-think log levels and make sure that no user-induced actions would be logged as `INFO` to ensure `INFO` would be safe from disk-flooding DoS attacks.