walletrpc: add debug log statement for invalid PSBT

To help debug remote signing issues, it's helpful to get the raw PSBT
that failed to be parsed. This is necessary since serializing an invalid
PSBT is allowed and the checks only fail when trying to de-serialize
such an invalid packet.

lnrpc/walletrpc/walletkit_server.go

@@ -1224,6 +1224,8 @@ func (w *WalletKit) SignPsbt(_ context.Context, req *SignPsbtRequest) (
 		bytes.NewReader(req.FundedPsbt), false,
 	)
 	if err != nil {
+		log.Debugf("Error parsing PSBT: %v, raw input: %x", err,
+			req.FundedPsbt)
 		return nil, fmt.Errorf("error parsing PSBT: %v", err)
 	}