lnworker: nicer logs/error msgs for payment failure

try to decode onion error and log it in human-readable form

electrum/lnonion.py

@@ -25,7 +25,7 @@
 
 import io
 import hashlib
-from typing import Sequence, List, Tuple, NamedTuple, TYPE_CHECKING
+from typing import Sequence, List, Tuple, NamedTuple, TYPE_CHECKING, Dict, Any, Optional
 from enum import IntEnum, IntFlag
 
 from . import ecc
@@ -33,7 +33,7 @@ from .crypto import sha256, hmac_oneshot, chacha20_encrypt
 from .util import bh2u, profiler, xor_bytes, bfh
 from .lnutil import (get_ecdh, PaymentFailure, NUM_MAX_HOPS_IN_PAYMENT_PATH,
                      NUM_MAX_EDGES_IN_PAYMENT_PATH, ShortChannelID, OnionFailureCodeMetaFlag)
-from .lnmsg import OnionWireSerializer, read_bigsize_int, write_bigsize_int
+from .lnmsg import OnionWireSerializer, read_bigsize_int, write_bigsize_int, UnknownMsgType
 
 if TYPE_CHECKING:
     from .lnrouter import LNPaymentRoute
@@ -431,7 +431,7 @@ class OnionRoutingFailure(Exception):
         try:
             failure_code = OnionFailureCode(failure_code)
         except ValueError:
-            pass  # uknown failure code
+            pass  # unknown failure code
         failure_data = failure_msg[2:]
         return OnionRoutingFailure(failure_code, failure_data)
 
@@ -440,6 +440,13 @@ class OnionRoutingFailure(Exception):
             return str(self.code.name)
         return f"Unknown error ({self.code!r})"
 
+    def decode_data(self) -> Optional[Dict[str, Any]]:
+        try:
+            message_type, payload = OnionWireSerializer.decode_msg(self.to_bytes())
+        except UnknownMsgType:
+            payload = None
+        return payload
+
 
 def construct_onion_error(
         reason: OnionRoutingFailure,