qt: follow-up LN/onchain req sep: consistent URI/bolt11 errors

- if we show error for LN request, we should also show error for LN-only bip21 URI
  follow-up af0ac40478
- for LN request, show better error msg if don't have any channels
  closes https://github.com/spesmilo/electrum/issues/9413

note: would be nice if qml GUI could reuse this logic (wallet.get_help_texts_for_receive_request)

electrum/wallet.py

@@ -3282,6 +3282,9 @@ class Abstract_Wallet(ABC, Logger, EventListener):
         ln_swap_suggestion = None
         ln_rebalance_suggestion = None
         URI = self.get_request_URI(req) or ''
+        lightning_has_channels = (
+            self.lnworker and len([chan for chan in self.lnworker.channels.values() if chan.is_open()]) > 0
+        )
         lightning_online = self.lnworker and self.lnworker.num_peers() > 0
         can_receive_lightning = self.lnworker and amount_sat <= self.lnworker.num_sats_can_receive()
         status = self.get_invoice_status(req)
@@ -3309,7 +3312,10 @@ class Abstract_Wallet(ABC, Logger, EventListener):
                 address_help = URI_help = (_("This address has already been used. "
                                              "For better privacy, do not reuse it for new payments."))
             if req.is_lightning():
-                if not lightning_online:
+                if not lightning_has_channels:
+                    ln_is_error = True
+                    ln_help = _("You must have an open Lightning channel to receive payments.")
+                elif not lightning_online:
                     ln_is_error = True
                     ln_help = _('You must be online to receive Lightning payments.')
                 elif not can_receive_lightning:
@@ -3321,6 +3327,10 @@ class Abstract_Wallet(ABC, Logger, EventListener):
                         ln_help += '\n\n' + _('You may have that capacity if you rebalance your channels.')
                     elif bool(ln_swap_suggestion):
                         ln_help += '\n\n' + _('You may have that capacity if you swap some of your funds.')
+                # for URI that has LN part but no onchain part, copy error:
+                if not addr and ln_is_error:
+                    URI_is_error = ln_is_error
+                    URI_help = ln_help
         return ReceiveRequestHelp(
             address_help=address_help,
             URI_help=URI_help,