Report payment error info in listpays

[sr-gi]

It looks like the current version of listpays does not return the decrypted error information in case a payment fails, only the undecrypted onion in erroronion. It would be really useful if the payment error is returned (or at least the error code corresponding to the reason) so failures could be tracked and acted on.

An example use case would be sending a payment and polling listpays to check the status of it, and report based on it. Currently, if the payment fails there is not much you could say about it.