trampoline: improve payment success

- better error handling: previously we stopped all attempts on any of
  TRAMPOLINE_EXPIRY_TOO_SOON, UNKNOWN_NEXT_PEER, TEMPORARY_NODE_FAILURE.
  Instead we should retry (but see code comments).
- previously payments failed if ALL of the following criteria applied:
  - sender is paying via trampoline, but not via the ACINQ node (which is
    special cased)
  - receiver only has private channels and has put r_tags into invoice, along
    with setting the trampoline feature bit in the invoice, however the receiver
    is not connected to any trampoline forwarders directly
  The sender would then assume that the private routing hints in the invoice
  correspond to trampoline forwarders.
- also, previously if both the sender and the recipient used trampoline and
  they shared a trampoline forwarder (that they were both connected to), the
  private channels the recipient had (with nodes other than the shared TF)
  would never be attempted.

electrum/lnchannel.py

@@ -1053,8 +1053,7 @@ class Channel(AbstractChannel):
                 # if we are forwarding, save error message to disk
                 if self.lnworker.get_payment_info(htlc.payment_hash) is None:
                     self.save_fail_htlc_reason(htlc.htlc_id, error_bytes, failure_message)
-                else:
-                    self.lnworker.htlc_failed(self, htlc.payment_hash, htlc.htlc_id, error_bytes, failure_message)
+                self.lnworker.htlc_failed(self, htlc.payment_hash, htlc.htlc_id, error_bytes, failure_message)
 
     def save_fail_htlc_reason(
             self,