Payjoin receiver times out when sender falls back

Before this commit, if a BIP78 sender rejected the
PSBT sent by the receiver in the negotiation, JM's
receiver was hanging as the fallback broadcast was not
triggered. This commit corrects that error, and also
makes the fallback message visible on the GUI, and makes
it more ambiguous (since it not always clear whether
the transaction was broadcast or not, in this case).

jmclient/jmclient/payjoin.py

@@ -650,7 +650,11 @@ def fallback_nonpayjoin_broadcast(err, manager):
     log.warn("Error message was: " + err.decode("utf-8"))
     original_tx = manager.initial_psbt.extract_transaction()
     if not jm_single().bc_interface.pushtx(original_tx.serialize()):
-        log.error("Unable to broadcast original payment. The payment is NOT made.")
+        errormsg = ("Unable to broadcast original payment. Check your wallet\n"
+        "to see whether original payment was made.")
+        log.error(errormsg)
+        # ensure any GUI as well as command line sees the message:
+        manager.user_info_callback(errormsg)
         quit()
         return
     log.info("Payment made without coinjoin. Transaction: ")
@@ -683,7 +687,6 @@ def process_payjoin_proposal_from_server(response_body, manager):
         log.error("Payjoin tx from server could not be parsed: " + repr(e))
         fallback_nonpayjoin_broadcast(b"Server sent invalid psbt", manager)
         return
-
     log.debug("Receiver sent us this PSBT: ")
     log.debug(manager.wallet_service.human_readable_psbt(payjoin_proposal_psbt))
     # we need to add back in our utxo information to the received psbt,
@@ -875,6 +878,12 @@ class PayjoinServer(Resource):
                                     "rejected from mempool.",
                                     "original-psbt-rejected")
 
+        # Now that the PSBT is accepted, we schedule fallback in case anything
+        # fails later on in negotiation (as specified in BIP78):
+        self.manager.timeout_fallback_dc = reactor.callLater(60,
+                                                fallback_nonpayjoin_broadcast,
+                                                b"timeout", self.manager)
+
         receiver_utxos = self.manager.select_receiver_utxos()
         if not receiver_utxos:
             return self.bip78_error(request,