rpcserver: add additional 'error' field in payment response

In order to not close the payment stream on payment error the additional
field have been added in payment response. Now error from stream Recv()
function means that something has happend inside the client and we unable
to process any payment farther, and error inside the payment response
means, that something wrong has happend with payment itself.

rpcserver.go

@@ -1239,7 +1239,18 @@ func (r *rpcServer) SendPayment(paymentStream lnrpc.Lightning_SendPaymentServer)
 				}
 				preImage, route, err := r.server.chanRouter.SendPayment(payment)
 				if err != nil {
-					errChan <- err
+					// If we receive payment error than,
+					// instead of terminating the stream,
+					// send error response to the user.
+					err := paymentStream.Send(&lnrpc.SendResponse{
+						PaymentError:    err.Error(),
+						PaymentPreimage: nil,
+						PaymentRoute:    nil,
+					})
+					if err != nil {
+						errChan <- err
+						return
+					}
 					return
 				}